The HTTP/2 spec allows Host to be used instead of :authority in
requests, and this is in fact *preferred* when converting from HTTP/1.

We erroneously treated Host as a connection header, thus disallowing
it in requests. The patch corrects this, aligning Node.js behaviour
with the HTTP/2 spec and with nghttp2:

 - Treat Host as a single-value header instead of a connection header.
 - Don't autofill :authority if Host is present.
 - The compatibility API (request.authority) falls back to using Host
   if :authority is not present.

This is semver-major because requests are no longer guaranteed to
have :authority set. An explanatory note was added to the docs.

Fixes: nodejs#29858