Bug 27857: Get the right column

Bug 27857: Get the right column

This patch makes the search for mandatory attribute types actually use
'code' instead of 'class' for comparisson.

Tests are added to cover this behavior.

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

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
https://bugs.koha-community.org/show_bug.cgi?id=27587

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>