Abort pending XHR in popstate handler

This fixes the bug where, if the user clicks back while a PJAX request is loading, the originally requested page is displayed anyway once the XHR response arrives.
I think this is a remnant from when I had the pushState delay and this change in the same branch. Because browsers go to the ``n-1``th page when you click back natively, you need to push something onto your history stack first if you're emulating that behaviour.