Fixes a bug when https://wpt.live/css/CSS2/positioning/abspos-001.xht
saved as file fails because we incorrectly recognized its MIME type
as HTML, leading to incorrect self-closing tag handling and thus
incorrect rendering.
This reverts my previous commit in WebServer and fixes the whole issue
in a much better way. Instead of having the MIME type guesser take a
URL (which we don't actually have in the WebServer at that point),
just take a path as a StringView.
Also, make use of the case-insensitive StringView::ends_with() :^)
