Change factorial function in combination

[bblodfon]

Hi @dpaukov ,

I've been using this version of the library and an issue arose, which I believe is minimal work to correct, so here it is:

I use the getNumberOfGeneratedObjects function which uses the simple factorial and not the one with the BigDecimal type, so when in my application I happened to use in the combination(n,k), n=21 and k=0 respectively, it return zero for the denominator. (factorial(21) > max Long).

If instead the Util.combination(n,k) function was using the BigDecimal factorial, and only after the calculation it converted to long type, that would an easy and satisfiable fix for me (^-^)

[dpaukov]

The change has been released in version 2.3 today.