fix(ngModel): revalidate the model when min/max expression values change for number inputs

As of this fix if the max or min value is changed via scope or by another ngModel
then it will trigger the model containing the min/max attributes to revalidate itself.

Closes #2404
fix(ngModel): consider ngMin/ngMax values when validating number input types

With this fix ngModel will treat ngMin as a min error and ngMax as a max error.
This also means that when either of these two values is changed then ngModel will
revaliate itself.