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

Note

This PR supersedes #1118, which I had to close after the original fork relationship was broken during repo visibility changes on my side. The code/content is the same contribution path, now resubmitted from a proper public fork.

[happysnaker]

Maintainer note: this patch intentionally targets the existing documented Host use case in RouteHeaders and keeps the change minimal by reading Request.Host, which is where Go promotes the HTTP Host header for normal server requests. Tests cover host matching with and without a port so behavior stays explicit.

[happysnaker]

Keeping this warm: the intent of this patch is still to make the documented RouteHeaders(...).Route("Host", ...) usage work with normal Go server requests by reading Request.Host, including the common host:port case. Tests cover exact-host and wildcard-host matching with and without a port, and the change stays local to middleware/route_headers.go.