fix($observe): check if the attribute is undefined

src/ng/compile.js

@@ -1073,7 +1073,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
 
         listeners.push(fn);
         $rootScope.$evalAsync(function() {
-          if (!listeners.$$inter) {
+          if (!listeners.$$inter && attrs.hasOwnProperty(key)) {
             // no one registered attribute interpolation function, so lets call it manually
             fn(attrs[key]);
           }

test/ng/directive/inputSpec.js

@@ -1331,6 +1331,11 @@ describe('input', function() {
     expect(scope.name).toEqual('adam');
   });
 
+  it('should not add the property to the scope if name is undefined', function() {
+    compileInput('<input type="text" ng-model="name" />');
+    expect(scope.form['undefined']).toBeUndefined();
+  });
+
   describe('compositionevents', function() {
     it('should not update the model between "compositionstart" and "compositionend" on non android', inject(function($sniffer) {
       $sniffer.android = false;