fix #860 FsCheck Failure in JsonParser
fix #860 FsCheck Failure in JsonParser

The JsonParser is working correctly. The test/property is expecting the
unicode literal within the string to be the unicode character.

add an unescape method and two tests to demonstrate that it is fixed.

AtwooTM/FSharp.Data,scrwtp/FSharp.Data,johansson/FSharp.Data,johansson/FSharp.Data,taylorwood/FSharp.Data,AtwooTM/FSharp.Data,tpetricek/FSharp.Data,taylorwood/FSharp.Data,tpetricek/FSharp.Data,scrwtp/FSharp.Data,ImaginaryDevelopment/FSharp.Data,johansson/FSharp.Data,ImaginaryDevelopment/FSharp.Data,olgierdd/FSharp.Data,olgierdd/FSharp.Data,tpetricek/FSharp.Data,olgierdd/FSharp.Data,AtwooTM/FSharp.Data,ImaginaryDevelopment/FSharp.Data,scrwtp/FSharp.Data,taylorwood/FSharp.Data