publish: Preserve the request connection header.

publish: Preserve the request connection header.

The Guile web server is reading the response connection header to decide
whether to close the connection. However, as the request connection header is
not forwarded to the response, this mechanism cannot work.

* guix/scripts/publish.scm (add-extra-headers): New procedure.
(make-request-handler): Use it to forward the request connection header to the
response.