fix(ngOptions): use reference check only when not using trackBy

Change the check on changed value for ngOptions to ensure that a reference check is only done
when trackBy is not used.  Previously, if trackBy was being used but the values of the objects were
equal a reference check was then used which would cause the check to be true if the objects had
different memory references but the values were equal.  This can cause issues with forms being
marked dirty when the value of the model as not actually changed.

Closes angular#11936

Author: Steven Easter

src/ng/directive/ngOptions.js

@@ -707,7 +707,7 @@ var ngOptionsDirective = ['$compile', '$parse', function($compile, $parse) {
         if (!ngModelCtrl.$isEmpty(previousValue)) {
           var nextValue = selectCtrl.readValue();
           if (ngOptions.trackBy && !equals(previousValue, nextValue) ||
-                previousValue !== nextValue) {
+              !ngOptions.trackBy && previousValue !== nextValue) {
             ngModelCtrl.$setViewValue(nextValue);
             ngModelCtrl.$render();
           }

test/ng/directive/ngOptionsSpec.js

@@ -1176,6 +1176,28 @@ describe('ngOptions', function() {
 
     });
 
+    it('should not set view value again if the tracked property of the model has not changed when using trackBy', function() {
+
+      createSelect({
+        'ng-model': 'selected',
+        'ng-options': 'item for item in arr track by item.id'
+      });
+
+      scope.$apply(function() {
+        scope.selected = {id: 10, label: 'ten'};
+      });
+
+      spyOn(element.controller('ngModel'), '$setViewValue');
+
+      scope.$apply(function() {
+        scope.arr[0] = {id: 10, label: 'ten'};
+      });
+
+      // update render due to equality watch
+      expect(element.controller('ngModel').$setViewValue).not.toHaveBeenCalled();
+
+    });
+
     it('should not re-render if a property of the model is changed when not using trackBy', function() {
 
       createSelect({