MOISTURE-RESISTANT PHOSPHOR COMPOSITIONS AND ASSOCIATE METHODS BACKGROUND [0001] The invention relates generally to red-emitting phosphors, and particularly moisture-resistant red-emitting phosphors. More particularly, the invention relates to moisture-resistant K2SiF6:Mn4+, and methods of making the same. [0002] Red-emitting phosphors based on complex fluoride materials activated by Mn4+, such as those described in US 7,358,542, US 7,497,973, and US 7,648,649, can be utilized in combination with yellow/green emitting phosphors such as YAG:Ce or other garnet compositions to achieve warm white light (CCTs<5000 K on the blackbody locus, color rendering index (CRI) >80) from a blue LED, equivalent to that produced by current fluorescent, incandescent and halogen lamps. These materials absorb blue light strongly and efficiently emit between about 610-635 nm with little deep red/NIR emission. [0003] While the efficacy and CRI of lighting systems using Mn4+ doped fluoride hosts can be quite high, one potential limitation is their susceptibility to degradation under use conditions. It is possible to reduce this degradation using post-synthesis processing steps, as described in US 8,252,613. However, development of these materials with improved performance and stability is desirable. BRIEF DESCRIPTION [0004] In one aspect, the present invention relates to a phosphor composition including particles of K2SiF6:Mn4+ coated with a manganese-free complex fluoride. The manganese-free complex fluoride includes a composition of formula I: A3[MF6], where A is selected from Na, K, Rb and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, Gd and combinations thereof. The composition of formula I:A3[MF6] has lower water solubility than a water solubility of K2SiF6. [0005] In one aspect, a lighting apparatus according to the present invention includes a light source and a phosphor composition radiationally coupled to the light source. The phosphor composition includes particles of K2SiF6:Mn4+ coated with a manganese-free complex fluoride. The manganese-free complex fluoride includes a composition of formula I: A3[MF6], where A is selected from Na, K, Rb and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, Gd and combinations thereof. The composition of formula 1:A3[MF6] has lower water solubility than a water solubility of K2SiF6. [0006] In another aspect, the present invention relates to a method for preparing a phosphor composition. The method includes combining a saturated solution of a manganese-free complex fluoride including a composition of formula I: A3[MF6], where A is selected from Na, K, Rb, and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, Gd and combinations thereof, with K2SiF6:Mn41" phosphor in solid form to form a slurry, wherein the composition of formula I:A3[MF6] has a water solubility lower than a water solubility of K2SiF6. The method further includes filtering the slurry, and isolating a product from the slurry. [0007] In one aspect, the present invention relates to a phosphor composition derived from combining K2SiF6:Mn4+ in solid form with a saturated solution of a manganese-free complex fluoride including a composition of formula I:A3[MF6], where A is selected from Na, K, Rb, and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, Gd, and combinations thereof, wherein the composition of formula I:A3[MF6] has a water solubility lower than a water solubility of K2SiF6. DRAWINGS [0008] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein: [0009] FIG. 1 is a micrograph of K2SiF6:Mn4+ phosphor composition. [0010] FIG. 2 is a micrograph of a phosphor composition containing particles of K2SiF6:Mn4+ coated with a manganese-free complex fluoride of formula I, in accordance with one embodiment of the invention; [0011] FIG. 3 is a schematic cross-sectional view of a lighting apparatus, in accordance with one embodiment of the invention; [0012] FIG. 4 is a schematic cross-sectional view of a lighting apparatus, in accordance with another embodiment of the invention; [0013] FIG. 5 is a schematic cross-sectional view of a lighting apparatus, in accordance with yet another embodiment of the invention. DETAILED DESCRIPTION [0014] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about," is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. In the following specification and claims, the singular forms "a", "an" and "the" include plural referents, unless the context clearly dictates otherwise. [0015] As used herein, the term "phosphor", "phosphor composition" or "phosphor material" may be used to denote both a single phosphor composition, as well as a moisture-resistant phosphor composition, and in some embodiments, a coated phosphor composition. [0016] According to one embodiment of the invention, a phosphor composition is derived from combining K2SiF6:Mn4+ in solid form with a saturated solution of a manganese-free complex fluoride. The manganese-free complex fluoride includes a composition of formula I:A3[MF6], where A is selected from Na, K, Rb and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, Gd, and combinations thereof. In one embodiment, a water solubility of the composition of formula I:A3[MF6] is lower than a water solubility of K2SiF6. Inventors believe that the resulting phosphor composition includes coated particles of K2SiF6:Mn4+ with a manganese-free complex fluoride and thus provide high resistance to moisture-induced degradation. FIGS. 1 and 2, respectively, show micrographs of as prepared K2SiF6:Mn4+ (known in the art) and the phosphor composition produced by combining K2SiF6:Mn4+ with a saturated solution 1 including K3A1F6 as described below with respect to an example. It is clear that the micrograph in FIG. 2 shows substantially coated particles. [0017] It is believed that the phosphor composition may have a core-shell structure. Substantially all of the particles of a core phosphor i.e. K2SiF6:Mn4+ may be coated with a manganese-free material (also referred to as "shell phosphor"). Advantageously, the coating may have significantly less degradation under high temperature and high humidity conditions, as compared to the core particles, thereby protecting the core particles from the effects of atmospheric moisture. In preferred embodiments, every particle may be covered with the manganese-free complex fluoride. However, if a small number of particles do not become fully covered under processing conditions, the overall characteristics of the phosphor would not be adversely affected for most applications. [0018] As used herein, a complex fluoride is a coordination compound, containing at least one coordination center (for example "M" in the examples above), surrounded by fluoride ions acting as ligands, and charge-compensated by counter ions (e.g. "A" or "E" in the examples above), as necessary. These complex fluorides may further include an activator ion, for example manganese ion (Mn4+), and may also be referred as manganese-doped fluoride phosphor. The activator ion (Mn4+) also acts as a coordination center, substituting part of the centers of a host lattice, e.g., M. The host lattice (including the counter ions) can further modify the excitation and emission properties of the activator ion. [0019] A variety of manganese-free complex fluorides can be used for coating individual particles of the manganese-doped core fluoride that is K2SiF6:Mn4^. Furthermore, use of a host composition (for example, K2SiF6) of the core material may be a desirable choice for the manganese-free coating to avoid mismatching of various features, such as lattice parameters, and refractive index, which otherwise may adversely affect the emission properties of the resulting coated phosphor composition. However, K2SiF6 may not be suitable coating material for coating K2SiF6:Mn4+ because of its solubility/reactivity with water. K.2SiF6 may react with water, in particular, and degrade under a humid atmosphere. [0020] As used herein, "water solubility" or "solubility with water" refers to solubility as well as reactivity of a composition with water. In one embodiment, a composition may dissolve in water. In another embodiment, a composition may chemically react with water. For example, K2SiF6 may not dissolve in water, instead may react with water and produce some precipitates/products. [0021] A number of manganese-free complex fluorides of formula I:A3[MF6], where A is selected from Na, K, Rb, Cs, Li, Mg, Ag and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, La, a lanthanide, Bi, and combinations thereof, have been studied for their compatibility to combine with the K2SiF6:Mn4+ phosphor, for example lattice matching with the host material K2SiF6, and their solubility in water to analyze their moisture resistivity. [0022] Table 1 shows some examples of manganese-free complex fluoride compositions of formula 1 along with their lattice parameter and solubility in water (measured as described below with respect to examples). It has been observed that the composition samples 2-7 have lattice parameters comparative with that of K2SiF6, and may have acceptable lattice matching with K2SiF6. Furthermore, samples 2,4 and 7 have lower solubility in water then K.2SiF6. Table 1 [0023] Some embodiments of the invention thus provide a phosphor composition derived from combining K2SiF6:Mn4+ in solid form with a saturated solution of a manganese-free complex fluoride including a composition of formula I:A3[MF6], where A is selected from Na, K, Rb, and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, Gd, and combinations thereof. A may further include Li, Ag, or a combination thereof in about 50 percent amount. [0024] In some embodiments, a phosphor composition includes K2SiF6:Mn4+ particles coated with a manganese-free complex fluoride includes a composition of formula I:A3[MF6], where A is selected from Na, K, Rb, and combinations thereof; and M is selected from Al, Ga, In, Sc, Y, Gd, and combinations thereof. In certain embodiments, the manganese-free fluoride includes a composition of formula I: A3[A1F6], where A is selected from Na, K, and combinations thereof. Moreover, in some embodiments, the manganese-free complex fluoride composition of formula I:A3[MF6] having a lower solubility in water than the solubility of K2SiF6 in water is desirable. For example, K3A1F6 and K2NaAlF5 have lower solubility in water than the solubility of K2SiF6 in water. [0025] In some embodiments, the manganese-free complex fluoride may further include K2SiF6. An amount of K2SiF6 may be added while preparing a saturated solution of the A3[MF6] for coating the K2SiF6 particles that is described in details below. [0026] A variety of methods may be used to prepare manganese-doped or manganese-free fluoride compounds, depending on different starting materials, methods to provide a manganese activator in the proper oxidation state, and the like. Some of preferred methods are described in U.S. Patent 7,497,973. [0027] The moisture-resistant coated K2SiF6:Mn4+ phosphor composition provided by embodiments of the present invention has an intense red luminescence property for electromagnetic excitations corresponding to the various absorption fields of the product. These phosphors may be desirably used in lighting or display systems. One embodiment of the invention is directed to a lighting apparatus that includes the phosphor composition radiationally coupled to a light source. [0028] A cross sectional view of a lighting apparatus or light emitting assembly or lamp 10 according to the present invention is shown in FIG. 3. Lighting apparatus 10 includes a semiconductor radiation source, shown as light emitting diode (LED) chip 12, and leads 14 electrically attached to the LED chip. The leads 14 may be thin wires supported by a thicker lead frame(s) 16 or the leads may be self-supported electrodes and the lead frame may be omitted. The leads 14 provide current to LED chip 1 and thus cause it to emit radiation. [0029] LED chip 12 may be any semiconductor blue or ultraviolet light source that is capable of producing white light when its emitted radiation is directed onto a phosphor. In particular, the semiconductor light source may be a blue emitting LED semiconductor diode based on a nitride compound semiconductor of formula lniGajAlkN (where 00); and Ca,.;^..^,, (Li,Na)xEivM1+<0.2). In particular, suitable phosphors for use in blends with the coated phosphor composition are (Ca, Ce)3Sc2Si3Oi2(CaSiG); (Sr,Ca,Ba)3Al1.xSix04+xF,.x:Ce3+ ((Ca, Sr, Ce)3(Al, Si)(0, F)5(SAS0F)); (Ba,Sr,Ca)2Sii^04-24:Eu2+ (wherein 0<^<0.2); (Y,Gd,Tb,La,Sm,Pr,Lu)3(Al,Ga)5-aOi2.3/2a:Ce3+ (wherein 0