Unify way of calling include_attribute? in attibutes and association methods

There is defined method for that: https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer.rb#L365

which already is used in: https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer.rb#L359