FORM 2 THE PATENTS ACT, 1970 5 (39 of 1970) & THE PATENT RULES, 2003 10 COMPLETE SPECIFICATION (See Section 10 and Rule 13) 15 Title of invention: METHOD FOR PREPARING POROUS SEPARATION MEMBRANE COMPRISING ELASTIC MATERIAL, POROUS SEPARATION MEMBRANE PREPARED BY SAID METHOD, AND SECONDARY BATTERY COMPRISING SAID SEPARATION MEMBRANE 20 APPLICANT: LG CHEM, LTD. 25 A Company incorporated in South Korea Having Address: 128, Yeoui-daero, Yeongdeungpo-gu, Seoul 150-721, Republic of Korea 30 The following specification particularly describes the invention and the manner in which it is to be performed. 1 TECHNICAL FIELD The present disclosure relates to a method of manufacturing a porous separator, and more particularly, to a method of manufacturing a porous separator including an elastic material, a porous separator manufactured by the method, and a secondary battery including the separator. The present application claims priorities to Korean Patent Application No. 10-2012-0106545 filed in the Republic of Korea on September 25, 2012, and Korean Patent Application No. 10-2013-0114158 filed in the Republic of Korea on September 25, 2013, the disclosures of which are incorporated herein by reference. BACKGROUND ART A secondary battery is a chemical battery that can be used semipermanently by continuously repeating charge and discharge using an electrochemical reaction, and can be classified into a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery, and a lithium secondary battery. Among them, a lithium secondary battery has a higher voltage and better energy density characteristics than the others, and thus is taking the lead in the secondary battery market. Also, depending on the type of an electrolyte, a lithium secondary battery can be divided into a lithium ion secondary battery using a liquid electrolyte and a lithium ion polymer secondary battery using a solid electrolyte. A lithium secondary battery includes a cathode, an anode, an electrolyte, and a separator, and among them, the separator plays a role of separating the cathode from the anode to electrically isolate the cathode and the anode and to improve the transfer of lithium ions or permeability based on a high porosity to increase ionic conductivity. As a polymer substrate of a generally used separator, a polyolefin-based polymer having advantageous properties for pore formation, chemical resistance, and excellent mechanical and thermal properties, such as polyethylene (PE), polypropylene (PP), and the like, is mainly used. A separator for a lithium secondary battery requires characteristics such as excellent permeability, low thermal shrinkage, high puncture strength, and the like; however, with the advancement of a high-capacity high-output battery, attempts to improve permeability are continuously being conducted. To manufacture a porous separator from polyolefin, a wet process that mixes polyolefin and a pore forming agent at high temperature, extrudes, and stretches, and subsequently, extracts the pore forming agent have been used. However, to improve permeability of a separator manufactured through the wet process, a way of increasing an amount of the pore forming agent, for example, a diluent, a plasticizer, and the like, have been used; however, as the content of the pore forming agent increases, stability of extrusion molding greatly reduces, difficulties occur since alterations have to be made to various process conditions including extrusion conditions, and environmental issues are raised due to a great amount of pore forming agents and solvents. Meanwhile, in contrast to a wet process using a solvent, a dry process without using a solvent enables mass production of a wide film and is more environmentally friendly than the wet process because there is no need for a solvent. However, a stretching process has a drawback in that a possibility of a short circuit occurring is high due to the tendency of a film shrinking in a direction opposite of its stretched direction. Accordingly, a film having passed through a stretching process has significant improvements in terms of mechanical properties such as a tensile strength in a stretched direction or a more stretched direction, but has a relatively low strength in a direction perpendicular to its stretched direction or a less stretched direction, so that the film may be torn out, resulting in a short circuit. Also, an internal short circuit may occur when the separator shrinks excessively due to increased temperature caused by overcharge or other reasons. Accordingly, there is still a demand for a porous separator in which a stretched film has improved strength in a stretched direction as well as a direction opposite to its stretched direction. DISCLOSURE Technical Problem The present disclosure is directed to providing a porous separator in which a film has a significant reduction in susceptibility to tears in a battery during or after a battery assembly process, and a method of manufacturing the same. Technical Solution To achieve the object, according to one aspect of the present disclosure, there is provided a porous separator including a mixture of an elastic material and a polymer resin at a content ratio of about 40:60 to about 5:95 based on a weight ratio, in which in the mixture, the elastic material is uniformly dispersed in the polymer, and a value of elongation at break in a low tensile strength direction at room temperature is greater than or equal to about 250%. According to another aspect of the present disclosure, there is provided a method of manufacturing a porous separator including forming an extruded sheet by extruding, through an extruder, a mixture of a polymer resin and an elastic material at a weight ratio of 95:5 to 60:40, forming a film by annealing and stretching the extruded sheet, and forming a porous separator by heat setting the stretched film. Advantageous Effects According to the present disclosure, a thermal shrinkage ratio of a film is reduced and an elongation at break is greatly increased, to provide a porous separator with improved stability. DESCRIPTION OF DRAWINGS FIG. 1 is a flowchart illustrating a process of preparing a porous separator according to an exemplary embodiment. MODE FOR DISCLOSURE Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the disclosure, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the disclosure. The term "permeability" as used herein represents a period of time during which an air of 100cc passes through a porous substrate, and as its unit, sec/100cc is used in the specification. The term "permeability" may be interchangeably used with the term "transmittance", and is generally indicated according to a Gurley value. The term "puncture strength" as used herein represents a resistance of a separator against external danger, for example, a passage of an external object, and "gram (g)" is used as its unit. The term "puncture strength" may be interchangeably used with the term "piercing strength" or "burst strength", and generally, a higher value lowers the risk of internal short circuits caused by a separator. The term "elongation at break" as used herein represents a ratio between a changed length and an initial length after a separator breaks when stretched at room temperature, and "%" is used as its unit. The elongation at break may be measured through a tensile test. A porous separator according to one aspect of the present disclosure includes a polymer resin in which an elastic material is uniformly dispersed. A content ratio between the elastic material and the polymer resin is from about 40:60 to about 5:95 or from about 30:70 to about 10:90 based on a weight ratio. When the elastic material is dispersed in the polymer resin within the content range, a value of elongation at break in a low tensile strength direction at room temperature may be about 250% or higher or about 300% or higher. For example, the polymer resin is a raw material particle for a separator provided between a cathode and an anode of a secondary battery to prevent a short circuit by maintaining an insulation condition, and as a non-limiting example, may be a polyolefin-based polymer resin. For example, the polyolefin-based polymer resin may be any one selected from polyethylene, for example, high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene, polypropylene, polybutylene, and polypentene, or combinations thereof, but is not limited thereto. In the present disclosure, generally, the elastic material represents a material having an elastic property that can be stretched up to more than double its original length under stress and return to its original length promptly when the applied stress is removed. The elastic material may include, but not limited to, an elastomer, a natural rubber, or a synthetic rubber. As a non-limiting example, the elastomer may include a polyolefin-elastomer (POE), a styrenic block copolymer (SBC), a vinyl chloride elastomer, a chlorinated polyethylene elastomer (CPE), a urethane elastomer (TPU), a polyester elastomer (TPEE), a polyamide elastomer (TPAE), a fluorinated elastomer, and a silicone elastomer. Among them, the polyolefin elastomer (POE) may be any one polymer selected from the group consisting of olefin, for example, ethylene, propylene, butylene, pentene, hexene, heptene, and octane, or two or more polymers, for example, a copolymer, a terpolymer, or a mixture thereof, or a copolymerized elastomer therewith, or a graft copolymer in which one species of monomer selected from a group consisting of ethylene, propylene, butylene, pentene, hexene, heptene, and octane, has a backbone chain structure and different species of monomer is grafted thereto in a form of a branch. According to an embodiment of the present disclosure, the polyolefin elastomer may be an ethylene-octane copolymer. According to another embodiment of the present disclosure, the polyolefin elastomer may be a copolymer, a terpolymer, a block copolymer, or a graft copolymer polypropylene, including polypropylene in a main chain. According to one embodiment of the present disclosure, the polyolefin elastomer has a melting temperature (Tm) of 90℃ to 165℃. When the melting temperature is 90℃ or less, heat resistance is reduced, making it unsuitable for use as a separator of an electrochemical device. The porous separator according to one embodiment of the present disclosure is uniaxially stretched, for example, in a machine direction (MD), or biaxially stretched. Here, the uniaxial stretch represents stretching a film in one direction, and the biaxial stretch represents stretching a film in two directions approximately perpendicular to one another. This stretching process is performed to form pores in the separator and provide strength as well. However, the stretched separator has a tendency to shrink in an opposite direction to its stretched direction. Particularly, as the temperature increases by an internal or external factor of the battery, the separator may further shrink, which may cause a short circuit of the battery and the like. By this reason, in a case of uniaxial stretch, the separator may have a longer length than those of both electrodes placed at both sides of the separator such that a surplus separator extends beyond the edges of the both electrodes while considering a stretched direction or a consequent shrink direction. Similarly, in a case of biaxial stretch, the separator may have a longer length than those of both electrodes in a more stretched direction or a consequent shrink direction. According to another aspect of the present disclosure, a secondary battery including a cathode, an anode, and the above-described separator interposed between the cathode and the anode is provided. Particularly, the secondary battery may include a lithium secondary battery including, for example, a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery. The cathode and the anode may be easily produced by a process and/or a method known in the art pertaining to the present disclosure. The cathode is produced in a manner of binding a cathode active material to a cathode current collector by a traditional method known in the art. In this instance, the cathode active material may be a typical cathode active material that can be commonly used in a cathode of an electrochemical device, and as a non-limiting example, includes LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiaCobMnc)O2 (0 < a < 1, 0 < b < 1, a + b + c = 1), LiNi1-YCoYO2, LiCo1-YMnYO2, LiNi1-YMnYO2 (here, 0 ≤ Y < 1), Li(NiaCobMnc)O4 (0 < a < 2, 0 < b < 2, a + b + c = 2), LiMn2-ZNiZO4, LiMn2-ZCoZO4 (here, 0