Make OCClassMockObject (and OCPartialMockObject) return the mocked class...

Make OCClassMockObject (and OCPartialMockObject) return the mocked class from its -class method.  We had previously made the partial mocked object do this, but the mock proxy itself really should do the same (it already implements the -isKindOfClass: method, which -class should probably follow).  This will help when regular code calls the -class method when a mock object is passed in.

Added a -proxyClass method for when the proxy's actual class is needed, which was in several places in the code.