Fix for min/max length type="number" input fields.

Added toString() method to the min/max length value comparison so that the validator will catch number lengths correctly, should also allow for catching of any other data types that are passed through to $validators as all object types have toString() prototype. Previous behaviour had angular reporting validity errors no matter what was entered in to a type="number" input field which had specified ng-minlength or ng-maxlength directives.