forms: Fix interpretation of custom list data

Prior to osTicket v1.9.4, drop down lists only allowed for a single entry.
After upgrading to 1.9.4 the database data for drop downs changed to allow
for multiple entries. However, the old data format was not properly handled
by the SelectionField::to_php() method which handled interpreting and
upgrading the database data.
Merge pull request #1425 from greezybacon/issue/dropdown-value-upgrade

forms: Fix interpretation of custom list data

Reviewed-By: Peter Rotich <peter@osticket.com>