2.5 Cookies serializer
Applications created before Rails 4.1 uses Marshal to serialize cookie values into the signed and encrypted cookie jars. If you want to use the new JSON-based format in your application, you can add an initializer file with the following content:
Rails.application.config.action_dispatch.cookies_serializer = :hybrid
cookieのserializerがrubyのMarshalからJSONに変わったって部分。
これcookieがバージョンまたいでないと発生しないからテスト面倒だねえ・・・。