Added description and unit tests to IO_ExtensionMethods_Folders.cs ( https://github.com/o2platform/FluentSharp/issues/10)

I tried to add also extra checks for some functions, hope it is ok . There is also a fix for a problem in String_ExtensionMethods.cs which I missed in the previous pull request.


on cases like this (where the methods are the same) we just check that they are the same, which is interesting since this is easy to do in Javascript, but not in C#

Maybe we could use the AST parsing methods to confirm the mappings

Anyway, the key point is that we shouldn't have repeated unittests for extension methods that are using the same code