Bug 18961: Use exact match for select filters on item search

Bug 18961: Use exact match for select filters on item search

The SQL operator LIKE is always used for filters when searching for
items.

If the filter is a select, we should search for an exact match.
That way we avoid problematic search like "%NFIC%" and "%FIC%" (one
includes the other one).

Test plan:
- Make sure you have collection codes 'Fiction' and 'Non-fiction'
- Do an item search
- Filter column 'Collection', select 'Fiction'
- Result: Column contains items from Fiction only

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>