Default Responder doesn't care about whether delete failed or not

With the current Responder there is no way to tell whether API call to
delete failed or not as it always returns OK status [1]. I would
prefer it to check the deletion status with the AR#destroyed? method
and reply with :unprocessable_entity when that return false. Should
such behavior be included in default Responder, WDYT?

[1] http://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/responder.rb#L171

destroyed? is not part of ActiveModel API. We could check persisted?
but the official way to check if an action was concluded with success
or not is by checking if errors are empty or not.