support jsonp response format used by ExpressJS

https://github.com/strongloop/express/blob/master/lib/response.js

https://github.com/strongloop/express/commits/master/lib/response.js
https://github.com/strongloop/express/commit/e218377a3d9d5c37790823f4e346a4f83b733e7d
https://github.com/strongloop/express/commit/f684a64df71a08c0caf2371aa305411df690a10f

  * jsonp response format (legacy):
		cb && cb(json);
  * jsonp response format (after commit #e218377, Oct/15/2013):
		typeof cb === 'function' && cb(json);
  * jsonp response format (after commit #f684a64, Jul/10/2014):
		/**/ typeof cb === 'function' && cb(json);