Bug 29182: Make sure methods call the overloaded ->store

Bug 29182: Make sure methods call the overloaded ->store

This patch addresses the issue of `request()` and `set_pending()`
calling $self->SUPER::store instead of the overloaded one.

This prevents created_on being set.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/ArticleRequest.t
=> FAIL: created_on is not set on two methods
3. Apply this patch
4. Repeat 2
=> SUCCESS: created_on is now set!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>