solved problems with mock_redis when used with Sinatra

When the sinatra gem is used, it mixes in Sinatra::Delegator for all classes. Thus set/get operations will fail with mock_redis since these calls will be handled by Sinatra and not mock_redis. This fix undefines the set/get methods and makes sure the wrapper's method_missing methods are used instead.

This way sinatra and mock_redis can co-exist and work as expected.