Related endpoints for STI resources do not yield proper types

Following up on #507

Given a model Animal with subclasses Cat and Dog, types get returned properly when querying directly for animals - e.g. hitting endpoint "api/animals" responds with types "cats" and "dogs" in the payload.

However, this behavior is not consistent when hitting the related endpoints of a resource - e.g. hitting "api/zoos/7/animals" responds with only type "animals" in the payload.

Currently, I am including "type" in the attributes object, which raises a warning and also breaks the JSON API specs. Is there a better workaround for this?

api/animals と api/zoos/7/animals を比較して、Animal のサブクラス Cat と Dog で STI の動作を再現します。まず関連リソースのエンドポイント処理を追跡してください。関連レスポンスが cats と dogs を有効な JSON:API タイプとして返し、attributes に type を追加せず、警告も発生させなければ完了です。