publish: Use 'x-raw-file' internal response header.

This adjusts the workaround for <http://bugs.gnu.org/21093> so that it's
not limited to a single content-type.

* guix/scripts/publish.scm (render-nar/cached): Add the 'x-raw-file'
header on the response.
(render-content-addressed-file): Likewise.
(with-content-length): Remove the 'x-raw-file' header.
(http-write): Instead of dispatching on 'application/octet-stream',
check whether 'x-raw-file' is set to determine whether to spawn a
thread.