Allow overriding the adapter with render option

Renamed config.adapter to config.default_adapter
Undid the rename, this request is just for the adapter option to render now.
@ggordon you're testing the `adapter: :json_api` option everywhere. We want to still not need to put it, but use a default adapter and override only where need.

Could you leave some tests where JSONAPI is the default adapter and you don't pass the `adapter: :json_api`? We need to assert that :json_api as default adapter will work.