Bug 37794: Fix form that POSTs without an op in Holds to pull

Bug 37794: Fix form that POSTs without an op in Holds to pull

We intend not to have forms with method="post" without an op variable (so we
can check that the op starts with "cud-" as part of the CSRF protection), but
because of bug 37728 some were missed.

In Holds to pull that's the form which lets you change from the default
starting and ending date. Switching that to a GET at least lets you refresh
the page without getting a browser warning about resending a POST and maybe
having your credit card double-charged.

Test plan:
1. Without the patch, Circulation - Holds to pull - change the start date to
   something earlier and click Submit
2. Refresh the page, get a warning about resubmitting data
3. Apply patch, Circulation - Holds to pull - change the start date to
   something earlier and click Submit
4. Refresh the page, no warning

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