Avoid setting withCredentials to null. Fixes #295. Fixes #279.

This should fix problems with IE6-8 when "Enable native XMLHTTP support" is disabled. For more details see https://github.com/jquery/jquery/pull/751#issuecomment-11837921

This will fix rails_ujs compatibility with existing jQuery versions so that we don't have to wait while it is fixed in jQuery side and avoid IE headaches for rails_ujs users.