fix(modelCtrl.$render) Change modelCtrl.$viewValue to $modelValue

Binding date model would not initialize datepicker correctly.

$viewValue: Actual string value in the view (as such, would never get to the object .startDate)
$modelValue: The value in the model, that the control is bound to.

Eval of $viewValue would always return string value, or in the case of an object [object]
```
if modelCtrl.$modelValue.startDate == null
                return el.val('')

            return el.val(_formatted(modelCtrl.$modelValue))
```