Unit test a service which returns ngResource in angularjs with mocha, chai and sinon

I have a service which returns $resource as like below

myApp.factory('myService', ['$resource', function($resource){
    return $resource('/myapi/:id', {id:'@_id'},
    {
        update:{
            method:'PUT',
            url:'myapi/update/url/:myId'
        }
    }
}]);

and the controller which uses the service as like below

myApp.controller('myController', ['$scope', 'myService', '$state', function($scope, myService, $state){

    $scope.isCallbackCalled = false;

    $scope.update = function() {
         return myService.update({myId:'4345'}, 
                    function(response){
                        // Couldn't able to test this success callback in mocha and sinon
                        $scope.isCallbackCalled = true;
                        $state.go('success');
                    }).$promise;
    };
});

and my unit test case as like below

describe('myController', function() {
    var myService,
        updateDefered,
        updateSpy,
        scope,
        myController,
        stateSpy;

    beforeEach(inject(function($controllers, $rootScope, $state, myService, $q) {
        scope = $rootScope.$new();
        updateDefered = $q.defer();
        updateSpy = sinon.stub().returns(updateDefered.$promise);
        stateSpy = sinon.stub();
        myService = {
            update:updateSpy
        };

        myController = $controller('myController', {
                           $scope:scope,
                           myService:myService,
                           $state:{
                               go:stateSpy
                           }
                       });

        updateDefered.resolve();
    }));

    it('should call scope.update() method and then update service', inject(function($rootScope, $httpBackend) {
        $httpBackend.whenPUT('/myapi/update/url/:myId').respond({});
        $rootScope.$apply();

        scope.update();
        $httpBackend.flush();
        // here it always fails, and I couldn't able to test the $resources
        // internal success callback
        expect(scope.isCallbackCalled).to.be.true;
        stateSpy.should.have.been.calledOnce;
    });
});

Here I couldn't able to unit test the isCallbackCalled when calling the $resource's update method with success callback. If I use as like below instead of internal success callback, i can able to unit test isCallbackCalled

$scope.update = function() {
         return myService.update({myId:'4345'}
                    // instead of internal success callback now I am using then method of promise 
                    //function(response){
                    //    Couldn't able to test this success callback in mocha and sinon
                    //    $scope.isCallbackCalled = true;
                    //    $state.go('success');
                    //}
                    ).then(function(response){
                        $scope.isCallbackCalled = true;
                        $state.go('success');
                    });
    };

So, How do I unit test when the resource method calling has internal success callback (whether it is called or not and changed the scope variabales) or how do I check whether or not stateSpy has been called?