[ControllerUnitTest] Make the controller class not to throw an exception for simple unit tests

[ianhong]

This is related to #1504. To make this scenario work, we should update the controller class as follows:

        // Do not override ViewData.Model unless passed a non-null value.
        if (ViewData != null && model != null )
        {
            ViewData.Model = model;
        }

        return new ViewResult()
        {
            ViewName = viewName,
            ViewData = ViewData,
            ViewEngine = _viewEngine,
        };

[yishaigalatzer]

Fixed with 2cf56c7