Bug 37021: Change item_id type as integer in holds endpoint

Bug 37021: Change item_id type as integer in holds endpoint

When one makes a GET call to holds endpoint and hold has
item attached to it, item_id is handled as string not integer
as it should.

To reproduce:
1. Make sure you have hold waiting in your database.
2. Call endpoint api/v1/holds/?q=[{"me.status":"W"}].
=> Note that holds item_id is displayed as item_id: "12345".
3. Apply this patch.
4. Call endpoint again.
=> item_id should now be displayed as 12345.

Also prove t/db_dependent/api/v1/holds.t.

Signed-off-by: Jan Kissig <jkissig@th-wildau.de>
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>