fix backbone test

backbone/backbone-tests.ts

@@ -105,6 +105,7 @@ function test_collection() {
     });
 
     var alphabetical = Books.sortBy(function (book) {
+        return null;
     });
 }
 

backbone/backbone.d.ts

@@ -49,7 +49,7 @@ declare module Backbone {
 
     export class ModelBase extends Events {
         fetch(options?: JQueryAjaxSettings);
-        url(): string;
+        url: any;
         parse(response);
         toJSON(): any;
     }