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.

src/Angular.js

@@ -751,9 +751,10 @@ function toJsonReplacer(key, value) {
  *
  * @param {Object|Array|Date|string|number} obj Input to be serialized into JSON.
  * @param {boolean=} pretty If set to true, the JSON output will contain newlines and whitespace.
- * @returns {string} Jsonified string representing `obj`.
+ * @returns {string|undefined} Jsonified string representing `obj`.
  */
 function toJson(obj, pretty) {
+  if (typeof obj === 'undefined') return undefined;
   return JSON.stringify(obj, toJsonReplacer, pretty ? '  ' : null);
 }
 

test/AngularSpec.js

@@ -879,5 +879,9 @@ 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);
+    });
   });
 });