Fixed bug for stubbed class methods

A class contains "+ (void)foo" method and B class is inherited from A one. Mocking B class, stubbing "+ (void)foo" and then removing all stubbs by calling "stopMocking" did not work as expected. Later calls to [B foo] native (non-stubbed) implementation failed.

Also, I've updated OCMockObjectClassMethodMockingTests test case to include test which reproduces this issue.

Please, let me know if there is anything wrong with this approach.