Bug 24464: Add K::A::Basket->creator

Bug 24464: Add K::A::Basket->creator

This patch adds a ->creator method to Koha::Acquisition::Basket to
retrieve the related Koha::Patron object.

Note: this should be re-done once 22658 is pushed, but at the time
being, there's not patch submitted, and it is not a trivial task because
of the column type missmatch. That said, the implementation of ->creator
in this class is simple enough to just rewrite it once 22658 is
submitted, and the tests will reflect the expected/unchanged behavior.

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

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>