fix(angular.toJson): only strip properties beginning with $$, not $

BREAKING CHANGE:

If you expected `toJson` to strip these types of properties before,
you will have to manually do this yourself now.

src/Angular.js

@@ -975,7 +975,7 @@ function bind(self, fn) {
 function toJsonReplacer(key, value) {
   var val = value;
 
-  if (typeof key === 'string' && key.charAt(0) === '$') {
+  if (typeof key === 'string' && key.charAt(0) === '$' && key.charAt(1) === '$') {
     val = undefined;
   } else if (isWindow(value)) {
     val = '$WINDOW';
@@ -996,7 +996,7 @@ function toJsonReplacer(key, value) {
  * @function
  *
  * @description
- * Serializes input into a JSON-formatted string. Properties with leading $ characters will be
+ * Serializes input into a JSON-formatted string. Properties with leading $$ characters will be
  * stripped since angular uses this notation internally.
  *
  * @param {Object|Array|Date|string|number} obj Input to be serialized into JSON.

test/AngularSpec.js

@@ -1131,8 +1131,13 @@ describe('angular', function() {
     });
 
 
-    it('should not serialize properties starting with $', function() {
-      expect(toJson({$few: 'v', $$some:'value'}, false)).toEqual('{}');
+    it('should not serialize properties starting with $$', function() {
+      expect(toJson({$$some:'value'}, false)).toEqual('{}');
+    });
+
+
+    it('should serialize properties starting with $', function() {
+      expect(toJson({$few: 'v'}, false)).toEqual('{"$few":"v"}');
     });