substitute: Avoid infinite loop when updating the substitute list.

Reported
at <http://lists.gnu.org/archive/html/guix-devel/2015-07/msg00119.html

>.

* guix/scripts/substitute.scm (http-multiple-get): When RESP has "Connection:
  close", consume HEAD anyway; always call PROC to read from BODY.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>