fix(input): ngList is updated when array model values are changed

fixes angular#1751

When the model referenced the same same array and the array values
where changed, the list wasn't updated.
Now watchCollection is used to detect changes in NgModelController.

isEmpty now detects empty arrays

BEHAVIOUR CHANGE
There is a change in the behaviour of ngList when typing a list.
When “a , b” is typed is automatically changed to “a, b”.

Author: Gonzalo Ruiz de Villa

src/ng/directive/input.js

@@ -385,7 +385,9 @@ var inputType = {
 
 
 function isEmpty(value) {
-  return isUndefined(value) || value === '' || value === null || value !== value;
+  return isUndefined(value) || value === '' || value === null ||
+    (value.length === 0 && isArrayLike(value)) ||
+    value !== value;
 }
 
 
@@ -1082,11 +1084,11 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
   // model -> value
   var ctrl = this;
 
-  $scope.$watch(function ngModelWatch() {
+  $scope.$watchCollection($attr.ngModel, function ngModelWatch(newValue, oldValue) {
     var value = ngModelGet($scope);
 
     // if scope model value and ngModel value are out of sync
-    if (ctrl.$modelValue !== value) {
+    if (!equals(ctrl.$modelValue, oldValue)) {
 
       var formatters = ctrl.$formatters,
           idx = formatters.length;

test/ng/directive/inputSpec.js

@@ -986,7 +986,7 @@ describe('input', function() {
       expect(scope.list).toEqual(['a']);
 
       changeInputValueTo('a , b');
-      expect(inputElm.val()).toEqual('a , b');
+      expect(inputElm.val()).toEqual('a, b');
       expect(scope.list).toEqual(['a', 'b']);
     });
 
@@ -1019,6 +1019,28 @@ describe('input', function() {
       changeInputValueTo('a,b: c');
       expect(scope.list).toEqual(['a', 'b', 'c']);
     });
+
+    it("should detect changes in the values of an array", function () {
+      var list = ['x', 'y', 'z'];
+      compileInput('<input type="text" ng-model="list" ng-list />');
+      scope.$apply(function() {
+        scope.list = list;
+      });
+      expect(inputElm.val()).toBe('x, y, z');
+      scope.$apply(function() {
+        list.unshift('w');
+      });
+      expect(inputElm.val()).toBe('w, x, y, z');
+    });
+
+    it('should be invalid if empty', function() {
+      compileInput('<input name="namesInput" ng-model="list" ng-list required/>');
+      changeInputValueTo('a');
+      expect(inputElm).toBeValid();
+      changeInputValueTo('');
+      expect(inputElm).toBeInvalid();
+    });
+
   });
 
   describe('required', function() {