Adds xhrFields to $.ajax - matches jQuery

I know matching jQuery isn't a design goal, but I like xhrFields over using beforeSend for setting withCredentials. If I know a server always needs cookies for CORS requests, I'd like to make a high-level $.ajaxSettings for xhrFields: {withCredentials: true} to apply to all CORS XHR requests while still allowing per-call use of beforeSend.