Bug 31183: Add Koha::Item::Transfers->filter_by_current

Bug 31183: Add Koha::Item::Transfers->filter_by_current

This patch introduces the aforementioned method.

To test:
1. Apply this unit tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Item/Transfers.t
=> FaIL: method not defined?
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>