A content provider without a length or chunked coding sends a body that
only the end of the connection delimits (RFC 9112 6.3), yet the response
offered keep-alive, so a keep-alive client waited for the server's idle
timeout. Send Connection: close and close after it, as Go's net/http
does. A Content-Length set by the handler is still dropped; the
overload that takes a length frames such a body.