Avoid inconsistent validity due to deleting from $error

We noticed that in some cases after clearing the field to type a new number the ng-invalid class stuck around despite ng-valid-international-phone-number. This fixes what may be due to an inconsistency in ngModelController after deleting the key from $error.

src/international-phone-number.coffee

@@ -90,7 +90,7 @@ angular.module("internationalPhoneNumber", []).directive 'internationalPhoneNumb
           ctrl.$setValidity 'international-phone-number', validity
         else
           value = ''
-          delete ctrl.$error['international-phone-number']
+          ctrl.$setValidity 'international-phone-number', true
         value