Bug 14636: Item search - fix search and sort by publication date for MARC21

Bug 14636: Item search - fix search and sort by publication date for MARC21

Problem: MARC21/NORMARC stores the publication
date in biblio.copyrightdate, but UNIMARC uses
biblioitems.publicationyear.

To test:
- Without patch in a MARC21 installation:
- Search by publication date
- Verify that the results don't match the
  publication year you searched for.
- Try sorting the table by publication year.
- Verify that the sort doesn't work.
- Apply patch.
- Repeat both, search and sort.
- Verify both work correctly now.
- Repeat tests on a UNIMARC installation.
- Verify both still work.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>

Tested searching and sorting successfully in a MARC21 installation. DID
NOT test in UNIMARC.

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

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

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>