[fhgffy]

Preserve Transfer-Encoding when forwarding an HTTP request body. The relay copies the original chunked bytes, but cleanHeaders removes their framing header, so the origin sees an empty request body instead of the upload.

A loopback regression test sends a chunked POST through the HTTP handler to a real HTTP server and checks that the origin receives the complete body.

When a transfer coding is present, remove any conflicting Content-Length before forwarding. Header serialization tests cover the conflict and verify that ordinary fixed-length requests keep their length.

Validation (Go 1.27.2, Windows/amd64): go test -p 1 ./... -count=1, go vet -p 1 ./..., and go build -p 1 ./....