fix $r->header_in() for "User-Agent", "Connection", and "Host" broken in r2005, r2006, and r2008.
fix $r->header_in() for "User-Agent", "Connection", and "Host"
broken in r2005, r2006, and r2008.

firebase/nginx,hy0kl/nginx,hy0kl/nginx,firebase/nginx,firebase/nginx,firebase/nginx,hy0kl/nginx