Fix type of "this" parameter in generic method body

[fcurdi]

When extracting a method, the field Parameters contains the this parameter if it is not static, but its type is the generic method (not instantiated). This is changed to the the instantiation of that method with its generic parameters. This means that if a method is within a type SomeType that has generic parameter T, in the method body this should be of that type but instantiated with the generic parameter T.