Break up serializer to serializable_hash mapping

This pull request breaks up the mapping of serializer class instantiation to `serializable_hash` / `as_json`. One method maps the serializer instances and the original `serializable_array` emits them as serializable hashes.

This is for those that wish to extend `ActiveModel::ArraySerializer` but only want to modify specific behaviour such as `serializable_array` but wish to take advantage of the helpful serializer instantiation.