Fix RouteHeaders middleware to handle Host header correctly

[nathannewyen]

Summary

Go's http server promotes the Host header to r.Host and removes it from the Header map. This means r.Header.Get("Host") always returns an empty string, making the Host-based routing example in RouteHeaders non-functional.

This change:

• Adds special handling for the "Host" header to read from r.Host instead of r.Header.Get("Host")
• Fixes the documentation example to avoid circular router references which would cause a stack overflow
• Adds comprehensive tests for Host header routing behavior

Changes

1. route_headers.go: Added conditional check for "host" header to use r.Host
2. route_headers.go: Updated example code in comments to work correctly
3. route_headers_test.go: Added tests covering Host header routing with subdomains and ports

Test Plan

• All existing tests pass
• New tests verify Host header is correctly read from r.Host
• Tests cover subdomain matching with wildcards
• Tests cover Host with port numbers

Fixes #691