Fixup querying drop-down lists via advanced search

It turns out that the _id field was not added to the cdata table when it was
created for SelectionField items, because the ::hasIdValue() method was not
implemented. This patch correctly searches SelectionFields via advanced
search.
Merge pull request #444 from greezybacon/issue/selection-field-ids

Fixup querying drop-down lists via advanced search

Reviewed-By: Peter Rotich <peter@osticket.com>