fix(angularjs): add toJSON method

angularjs/angular-resource.d.ts

@@ -136,6 +136,9 @@ declare module angular.resource {
         /** the promise of the original server interaction that created this instance. **/
         $promise : angular.IPromise<T>;
         $resolved : boolean;
+        toJSON: () => {
+          [index: string]: any;
+        }
     }
 
     /**