AngularJS : Testing factory that returns $http pro

Trying to test an angular service that returns an $http GET request and the then handler, but I'm not able to test that the logic actually works inside of the then function. Here is a basic, truncated version of the service code:

angular.module('app').factory('User', function ($http) {
  var User = {};

  User.get = function(id) {
    return $http.get('/api/users/' + id).then(function (response) {
      var user = response.data;
      user.customProperty = true;  
      return user;
    });
  };

  return User;
});

And here is the test:

beforeEach(module('app'));
beforeEach(inject(function(_User_, _$q_, _$httpBackend_, _$rootScope_) {
  $q = _$q_;
  User = _User_;
  $httpBackend = _$httpBackend_;
  $scope = _$rootScope_.$new();
}));

afterEach(function () {
  $httpBackend.verifyNoOutstandingRequest();
  $httpBackend.verifyNoOutstandingExpectation();
});

describe('User factory', function () {

  it('gets a user and updates customProperty', function () {
    $httpBackend.expectGET('/api/users/123').respond({ id: 123 });
    User.get(123).then(function (user) {
      expect(user.customProperty).toBe(true);  // this never runs
    });

    $httpBackend.flush();
  });

});

I feel like I've tried pretty much everything to test the logic in the then call, so if someone can offer suggestions I would greatly appreciate it.

Edit: my problem was also due to nonstandard injection practices, so the answer below worked outside of that.

A few things need to be changed

Use whenGET instead of expectGET in order to fake a response
In the test then callback, set the response to a variable available outside the callback so you can test it in an expect call
Make sure the expect call is outside any callbacks, so it always runs and shows any failures.

Putting it all together:

it('gets a user and updates customProperty', function () {
  $httpBackend.whenGET('/api/users/123').respond({ id: 123 });
  User.get(123).then(function(response) {
    user = response;
  })
  $httpBackend.flush();
  expect(user.customProperty).toBe(true); 
});

As can be seen at http://plnkr.co/edit/9zON7ALKnrKpcOVrBupQ?p=preview