feat(Angular.js): skip JSON.stringify for undefined

Return early in `angular.toJson` if the object to be stringified is `undefined`.
IE8 stringifies `undefined` to `'undefined'` whereas other browsers return
`undefined`. This normalizes behavior and passes currently broken unit tests
in IE8.

test/AngularSpec.js

@@ -909,6 +909,10 @@ describe('angular', function() {
     it('should not serialize scope instances', inject(function($rootScope) {
       expect(toJson({key: $rootScope})).toEqual('{"key":"$SCOPE"}');
     }));
+
+    it('should serialize undefined as undefined', function() {
+      expect(toJson(undefined)).toEqual(undefined);
+    });
   });
 
 });