Bug 22899: Add pending_hold method to Koha::Item

Bug 22899: Add pending_hold method to Koha::Item

To test:
 1 - Enable course reserves
 2 - Create a course
 3 - Add an item to the course
 4 - Attempt to view the course on the OPAC
 5 - Internal Server Error
 6 - Apply patch
 7 - Add an item to the holds queue by placing a hold and  running holds queue builder or:
     INSERT INTO tmp_holdsqueue (itemnumber) VALUES (###);
 8 - View the course page, note item appears 'Pending hold'
 9 - Remove the holdsqueue line
10 - View the course page, note item appears 'Available'
11 - prove -v t/db_dependent/Koha/Item.t

Signed-off-by: Hayley Mapley <hayleymapley@catalyst.net.nz>
I see Jonathan's comments about small improvements, but will sign off as
everything works as expected here.
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>