middleware: handle Host routing via Request.Host

[happysnaker]

Summary

• read Request.Host for RouteHeaders rules targeting Host
• document the net/http host promotion behavior in the middleware docs
• add coverage for host matching with and without an explicit port

Why

net/http promotes the HTTP Host header to Request.Host and removes it from Request.Header, so the existing Host-based example in RouteHeaders is misleading for normal browser traffic. This patch makes Host routing behave the way users expect and updates the docs/tests accordingly.

Closes #691

[happysnaker]

Friendly follow-up: this patch keeps the API surface unchanged, but makes Host-based routing line up with how net/http actually exposes host information (Request.Host instead of Request.Header["Host"]).

It also adds host-with-port coverage, which seems useful for local development cases like example.com:8080.

[happysnaker]

Resubmitted from a restored public fork as #1120. Closing this thread in favor of the new PR so the contribution stays reviewable from a proper fork relationship.