marshal_with should support inference of view function return type

It would be nice that when passing a schema class to the marshal_with decorator to have infer if serializing the response should be a list or single object (Schema(many = True or False)).

This would allow to to use MethodView's for APIs as suggested in http://flask.pocoo.org/docs/0.12/views/ . Where the "get" method can dispatch a single object or a list in a restful escenario.