feat(ngModelOptions): add allowInvalid option

This option allows to write invalid values to the model instead of having them become undefined.
Use this together with calling `ctrl.$setValidity` directly for displaying errors
from serverside validation.

Closes #8290
Closes #8313
Closes #9016

Author: shahata

src/ng/directive/input.js

@@ -1968,7 +1968,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
       if (!validatorPromises.length) {
         validationDone();
       } else {
-        $q.all(validatorPromises).then(validationDone);
+        $q.all(validatorPromises).then(validationDone, noop);
       }
     }
 

test/ng/directive/inputSpec.js

@@ -711,7 +711,7 @@ describe('NgModelController', function() {
       expect(ctrl.$pending).toBeUndefined();
     }));
 
-    it('should clear and ignore all pending promises when the input values changes', inject(function($q) {
+    it('should clear and ignore all pending promises when the input value changes', inject(function($q) {
       ctrl.$validators.sync = function(value) {
         return true;
       };