fix(NgModel): use string representation of the value in the ngMinlength and ngMaxlength

There is an issue - if ng-model value set to a nonstring (via js), the ngMinlength and ngMaxlength don't work properly.
Issue example http://plnkr.co/edit/y65LU7xJFSi4L9Ui4WqZ?p=preview

Fixes #7848