Fix to patching decimals in JsonMergePatch.SystemText

[mosteadman]

These are changes specific to the SystemText.JSON implementation of this framework

Developed to fix issue #38

I have added a JSONElement extension GetGenericNumber to the SystemText Patch Builder

Previously it would cast all JSON numbers to an Int64 (even if they were decimal)

Now, it will attempt to cast to an Int64 and failing that, will cast to a decimal instead

This should allow for patching of both integer and floating point numbers

I have also added a new test to demonstrate this working for both scenarios

@Morcatko - could you run your eyes over this and make sure I've not missed anything obvious ? Cheers

[Morcatko]

Very nice

[Morcatko]

I am quite busy these days, but expect a new nuget no later than this week