Bug 17783: Add Koha::Objects->single

Bug 17783: Add Koha::Objects->single

Returns one and only one object that is part of this set.
Returns undef if there are no objects found.

->single is faster than ->search->next

This is optimal as it will grab the first returned result without instantiating
a cursor.

It is useful for this Bug as we only want to select the top row of found issuing
rules.

To test:
1. Run t/db_dependent/Koha/Objects.t

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>