Assume Javascript delivery if XmlHttpRequest header set

The Symfony2 HttpFoundation library #1178246: Add Symfony2 HttpFoundation library to core makes it really easy to tell if an incoming request was sent using XmlHttpRequest. ($request->isXmlHttpRequest()). If it is, we should be able to automatically switch to a non-page delivery callback. (This strategy may change later, but this is still an improvement from where we are now.)