ember-cli-mirage response headers are missing from XHR object

Trying to understand the inconsistency in the response between using my api vs ember-cli-mirage.

I have a handler making a POST request to authenticate a user. The expected parameters for the handler are response, status and xhr. Using my API I receive what I'd expect- response is the data, status is the statusText and xhr is the xhr object. However using ember-cli-mirage everything comes under response (kind of), and status and xhr are both undefined.

Snippets of my code are below:

mirage/config.js

this.post(URI.AUTH_SIGN_IN, function(db, request) {
  const responseHeaders = {
    'access-token': 'abcxyz123',
    'client': 'foobarbaz',
    'token-type': 'Bearer',
    'expiry': '1497364419',
    'uid': 'user@example.com'
  };

  const user = {
    data: { id: 1, type: 'user', attributes: { uid: 'user@example.com', email: 'user@example.com', name: 'John Doe', provider: 'email' } }
  };

  return new Mirage.Response( 200, responseHeaders, user );
});

authenticators/devise.js

authenticate(identification, password) {
  ...
  this.makeRequest( credentials ).then(function(response, status, xhr) {
    // persists the five headers needed to send to devise-token-auth
    // with mirage; response = Response {type: "default", status: 200, ok: true, statusText: "OK", headers: Headers…}, status = undefined, xhr = undefined
    // with actual api; response = Object {data: Object}, status = "success", xhr = Object {readyState: 4, getResponseHeader: function, getAllResponseHeaders: function…}
   });
}

I believe I'm doing it all correctly, but I've been known to be wrong :). Any help is much appreciated.