fix(ngValue): correctly update the value property when value is undefined

Previously, when the expression evaluated to `undefined` the `value` property
was not updated. This happened because jqLite/jQuery's `prop(_, undefined)` is
treated as a getter, thus not apdating the property.

This commit fixes it by setting the property to `null` instead.
(On IE9 we use `''` - see inline comments for more info.)

Fixes #15603

Closes #15605

Author: frederikprijck

src/ng/directive/input.js

@@ -2142,7 +2142,10 @@ var ngValueDirective = function() {
    *  makes it possible to use ngValue as a sort of one-way bind.
    */
   function updateElementValue(element, attr, value) {
-    element.prop('value', value);
+    // Support: IE9 only
+    // In IE9 values are converted to string (e.g. `input.value = null` results in `input.value === 'null'`).
+    var propValue = isDefined(value) ? value : (msie === 9) ? '' : null;
+    element.prop('value', propValue);
     attr.$set('value', value);
   }
 

test/ng/directive/inputSpec.js

@@ -4211,6 +4211,26 @@ describe('input', function() {
       expect(inputElm[0].getAttribute('value')).toBe('something');
     });
 
+    it('should clear the "dom" value property and attribute when the value is undefined', function() {
+      var inputElm = helper.compileInput('<input type="text" ng-value="value">');
+
+      $rootScope.$apply('value = "something"');
+
+      expect(inputElm[0].value).toBe('something');
+      expect(inputElm[0].getAttribute('value')).toBe('something');
+
+      $rootScope.$apply(function() {
+        delete $rootScope.value;
+      });
+
+      expect(inputElm[0].value).toBe('');
+      // Support: IE 9-11
+      // In IE it is not possible to remove the `value` attribute from an input element.
+      if (!msie) {
+        expect(inputElm[0].getAttribute('value')).toBeNull();
+      }
+    });
+
     they('should update the $prop "value" property and attribute after the bound expression changes', {
       input: '<input type="text" ng-value="value">',
       textarea: '<textarea ng-value="value"></textarea>'