[MEDIUM] Stats: add support for the HEAD method

[MEDIUM] Stats: add support for the HEAD method

There are browsers which sometimes send HEAD requests to the stats
page, but it was not handled so it returned a 503 server error or
was simply sent to the default backend servers.

Now with a HEAD request, the stats return the headers and finish
there. Normally, other methods should be blocked so that the stats
page really catches the whole URI. Other methods would need to cause
a 405 Method not allowed to be returned.