Host requirements in semantics

[mnot]

A client &MUST; send a Host header field in all HTTP/1.1 request messages.
If the target URI includes an authority component, then a client &MUST;
send a field value for Host that is identical to that authority
component, excluding any userinfo subcomponent and its "@" delimiter
().
If the authority component is missing or undefined for the target URI,
then a client &MUST; send a Host header field with an empty field value.

These are all H1 specific; only the first one is explicitly called out as so. I think this entire block should be moved to messaging, possibly in a new section?

[mnot]

Dup of #182