datepicker $render with $viewValue

I'm not sure this is as simple as my Pull Request suggest, but I just updated Angular Strap on my project wich use another directive to convert the date on the equivalent timestamp (long) by adding a formatter to the ngModelController.

With controller.$modelValue, my formatter was ignored, with $viewValue, it's working fine.

A better solution will be to add native support of type number, I'll make another pull request if I find the time...