Testing express route methods

I'm trying to check whether res.json() is being called in a express get method.

However in my get method it waits for a promise before executing res.json();

Here is the controller method:

function get(req, res, next) {

    Service
        .doImportantThings()
        .then(success, error);

    function success(result) {
        res.json(result); // Method i want to test.
    }

    function error(error) {
        // Handles it
    }
}

Service:

function doImportantThings() {
    var deferred = q.defer();

    doStuff
        .then(success, error);

    function success(results) {
        deferred.resolve(output);
    }

    function error() {
        deferred.reject();
    }

    return deferred.promise;
}

Test:

    beforeEach(function () {
        var Service = require('../../../../app/services/service');
        ServiceMock = sinon.mock(Service);
        methodExpect = ServiceMock.expects('doImportantThings').returns(q.resolve("test"));
        res.json = sinon.spy();

        expressController = require('../../../../app/controllers/v1/controller');
    });

    afterEach(function() {
        ServiceMock.restore();
    });

    describe('get()', function () {

        it('should call res.json() one time', function () {
            expressController.get(req, res);    
            expect(res.json).to.have.been.calledOnce; // Fails 
        });

        it('should call res.json() with object argument.', function () {
            expressController.get(req, res)
            expect(res.json).to.have.been.calledWith("test"); // Fails


        });

    });

Because i'm using promises the expect always returns false. I've tried to use mocha's done() callback with no success. I have also tried to put a callback as a controller parameter and call done from there or do the expect there but the test either times out or doesn't assert.

All the answers i have found talk about using supertest which would probably work but i want to be able to test this without making a http request to the resource to do it.