I blindly ported the code from [ember-model's `ajax` function](https://github.com/ebryn/ember-model/blob/0.0.7/packages/ember-model/lib/rest_adapter.js#L127).

It seems that we can omit the `stringify` here because ember-auth [does the same thing](https://github.com/heartsentwined/ember-auth/blob/aa5b10364479f198ef2b2d1dd121aadd83e1cf36/lib/request/jquery.em#L25) in `Em.Auth.Request.Jquery#send`.

I'll send another pull request in a momemt.