Fixed issue with callback querystring failure

express/lib/response.js:238 failure when querystring contains > 1 callback function reference.

It is not uncommon for callback to appear in the querystring twice (albeit on accident) if jQuery's jsonp signature (e.g. jsonpCallback) is used incorrectly in conjunction with a manual querystring setting. The app should not fail just because of a querystring issue. This fixes that situation.

My test querystring which crashed the Node instance was "?callback=go&callback=go".