BUG/MINOR: h3: strengthen host/authority header parsing

BUG/MINOR: h3: strengthen host/authority header parsing

HTTP/3 specification has several requirement when parsing authority or
host header inside a request. However, it was until then only partially
implemented.

This commit fixes this by ensuring the following :
* reject an empty authority/host header
* reject a host header if an authority was found with a different value
* no authority neither host header present

This must be backported up to 2.6.