Centralize pass-through response relay handling #326

Several pass-through paths, including ProxyUpstream and ProxyFile, copy every upstream header to the downstream response and discard the error from io.Copy. Protocol handlers contain similar response-copy blocks.

This leaves three protocol cases inconsistent:

• Headers named by Connection, plus the standard hop-by-hop headers, can be forwarded downstream.
• Upstream trailers are not declared and copied to the downstream response.
• A chunked upstream response that ends early can be emitted as a cleanly terminated downstream response because the copy error does not abort the response.

A shared response relay helper could strip hop-by-hop headers, declare and copy trailers, respect HEAD and bodiless statuses, and abort the downstream response after a post-header copy failure. Copy failures should record the upstream URL, status, and byte count.

A route-level test can use an httptest upstream which returns a connection-scoped header, a trailer, and a truncated chunked body, then make a real client request through the proxy.

Observed on v0.8.1 and current main at 94c11b4.