Bug 37827: Switch OPAC download list form from POST to GET

Bug 37827: Switch OPAC download list form from POST to GET

The hidden form that the OPAC uses for downloading a list has no need to be
a POST, which is good because it can't be one without having an op param
that starts with "cud-" which it doesn't have.

Test plan:
1. Nothing visible will change, so start with the patch applied
2. Log in to the OPAC, then do a search for something like Perl which will
   return a couple of results. Select two, With selected titles: Add to
   list - New list - give it a name and Save
3. Lists - choose the one you just created
4. Download - ISBD
5. Verify that the page you were on didn't change, that a file was downloaded,
   and that the file contains the two titles in your list

Sponsored-by: Chetco Community Public Library
Signed-off-by: Chloe Zermatten <chloe.zermatten@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>