reverseproxy: Restore request's original host and header (fix #3509)
reverseproxy: Restore request's original host and header (fix #3509)

We already restore them within the retry loop, but after successful
proxy we didn't reset them, so as handlers bubble back up, they would
see the values used for proxying.

Thanks to @ziddey for identifying the cause.

abiosoft/caddy,l3dlp/caddy,mholt/caddy,l3dlp/caddy,l3dlp/caddy,abiosoft/caddy,mholt/caddy