Simplify @responses hash initialization
@responses hash needs to be initialized with mime types that we get from Collector#collect_mimes_from_class_level. Mime::Type class as key and nil as value. This need to happen before content negotiation. Before that, it was looping though mime types and executing mime-type-generated method inside collector (see AbstractController::Collector#generate_method_for_mime). That approach resulted in 2 unnecessary method calls for each mime type collected by Collector#collect_mimes_from_class_level. Now hash is initialized in place, without usage of Collector#custom method.