Bug 13562: Fix item search on item MARC subfields

Bug 13562: Fix item search on item MARC subfields

Test plan:
1. Add 4 item search fields (in Administration › Items search fields):
   a. 1 biblio field linked to a DB field
   b. 1 biblio field not linked to a DB field
   c. 1 item field linked to a DB field
   d. 1 item field not linked to a DB field
2. Make sure you have some data in those fields to search on.
3. Go to item search page and do a search using these new fields, make
   sure the result is correct.

Signed-off-by: Nick <nick@quecheelibrary.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit f70ad43f8fd49ad7fb43697c6953767e534f7368)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>