fix(required): correctly validate when required

docs/content/guide/di.ngdoc

@@ -284,6 +284,7 @@ Here is an example of using the injector service:
 var myModule = angular.module('myModule', []);
 ```
 
+Notice that `myModule`  depends upon no other modules as the second parameter is an empty array.
 Teach the injector how to build a `greeter` service. Notice that `greeter` is dependent on the
 `$window` service. The `greeter` service is an object that contains a `greet` method.
 

src/ng/directive/validators.js

@@ -68,9 +68,11 @@ var requiredDirective = ['$parse', function($parse) {
     require: '?ngModel',
     link: function(scope, elm, attr, ctrl) {
       if (!ctrl) return;
-      var value = attr.required || $parse(attr.ngRequired)(scope);
+      var value = 'required' in attr || $parse(attr.ngRequired)(scope);
 
-      attr.required = true; // force truthy in case we are on non input element
+      if (!attr.ngRequired) {
+        attr.required = true;
+      } // force truthy in case we are on non input element
 
       ctrl.$validators.required = function(modelValue, viewValue) {
         return !value || !ctrl.$isEmpty(viewValue);