reverseproxy: Set the headers in the replacer before `handle_response` (#4165)
reverseproxy: Set the headers in the replacer before `handle_response` (#4165)

Turns out this was an oversight, we assumed we could use `{http.response.header.*}` but that doesn't work because those are grabbed from the response writer, and we haven't copied any headers into the response writer yet.

So the fix is to set all the response headers into the replacer at a new namespace before running the handlers.

This adds the `{http.reverse_proxy.header.*}` replacer.

See https://caddy.community/t/empty-http-response-header-x-accel-redirect/12447
l3dlp/caddy,mholt/caddy,mholt/caddy,l3dlp/caddy,l3dlp/caddy