Bug 29082: Add filtering methods to Koha::ArticleRequests

Bug 29082: Add filtering methods to Koha::ArticleRequests

This patch adds handy methods for filtering Koha::ArticleRequests
resultsets.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/ArticleRequests.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>