RPValidatorPredicate was not so friendly with subclassing.

I am using RPJSONValidator, it's so far so good, however, I need to add some features to RPValidatorPredicate by subclassing, let's call it FooBarPredicate, when I call FooBarPredicate.isOptional, what I expect it to return is an instance of FooBarPredicate, actually it's a RPValidatorPredicate.