A request with more than one Host field line, or with a Host value
holding a byte that cannot appear in uri-host [ ":" port ], is now
answered with 400 (RFC 9112 §3.2). The handler sees only the first
Host line, so such a request could reach it with an authority that
differs from the one an intermediary acted on.
Only the byte set is checked, not the full grammar. An empty value
stays valid, and a request without a Host line is still accepted.
Close#2623