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.
The 1000ms is only called after the 500ms setTimeout in which it's nested - so it should end up firing 500ms after timeout.html would have loaded.

I did originally have a pjax:timeout handler calling preventDefault, but found it didn't matter – this test is going to fail anyway, with the same result, even if the fallback hard loads timeout.html.