Handle tokenKey injection for FormData

Hello,
using a custom ajax request eg.
```js
// ajax based upload

var fd = new FormData();
fd.append('fieldname', $('#file').get(0).files[0]);

Auth.ajax({
  url: '/test_upload',
  data: fd,
  contentType: false,
  cache: false,
  processData: false
});
```
ajax function in ember-auth will match 'object' case in switch and will add property to FormData instead it should add tokenKey with proper "append" method.

Hoping, could be useful