Fixed so classes with Guids datatype can be faked.

I was using FakeO in my project and I added a unique Id as a GUID and got the follow error.

System.TypeInitializationException : The type initializer for 'MyNamespace.FooTestData' threw an exception.
  ----> System.ArgumentException : Object of type 'System.Int32' cannot be converted to type 'System.Guid'.

The issue is now fixed, with both a test and the change.