UserReferenceConverter converts a WikiReference to UserReference

The UserReferenceConverter is implemented to convert anything to a UserReference by resolving the toString value of the given object.
This behaviour leads to unexpected result like its ability to convert a WikiReference to a UserReference.

This is problematic since an API might provide methods taking WikiReference and UserReference and velocity will call the one taking a UserReference since it's able to convert the reference to a UserReference.