feat($resource): allow props beginning with $ to be used on resources

BREAKING CHANGE:

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

src/ngResource/resource.js

@@ -493,6 +493,13 @@ angular.module('ngResource', ['ng']).
           shallowClearAndCopy(value || {}, this);
         }
 
+        Resource.prototype.toJSON = function () {
+          var data = extend({}, this);
+          delete data.$promise;
+          delete data.$resolved;
+          return data;
+        };
+
         forEach(actions, function (action, name) {
           var hasBody = /^(POST|PUT|PATCH)$/i.test(action.method);