EmberJS / Ember Data customize http header

using ember 1.13.2 with ember data 1.13.4 in conjunction with a rails backend.

according to the ember guides, the following statement should modify the http headers for every XHR request:

App.ApplicationAdapter = DS.RESTAdapter.extend({
  headers: {
    "Authorization": "secret key"
  }
});

so I added this as coffee script to my application

App.ApplicationAdapter = DS.RESTAdapter.extend
  headers: {
        "Authorization": "foofoo"
      }

and it's included and correctly compiled to javascript.

But the http header isn't extended with the new header at all.