Bug 26498: Set default expiration date for holds

Bug 26498: Set default expiration date for holds

Koha doesn't automatically set expiration date for holds
so they can live in the system forever. This patch adds
new sysprefs to control setting of default expiration
date for holds. Note that expiration date is only set
if input field for it is left blank.

To test:
1. Apply patch and update database.
2. Enable syspref "DefaultHoldExpirationdate" and set some values
to "DefaultHoldExpirationdatePeriod" and
"DefaultHoldExpirationdateUnitOfTime".
3. Add some holds for patron from staff client or OPAC and leave
expiration date input field blank.
=> Holds expiration date should be set according your settings
4. Disable "DefaultHoldExpirationdate" and repeat step 3.
=> Expiration date shouldn't be set.

Also prove prove t/db_dependent/Hold.t

Sponsored-by: Koha-Suomi Oy
Signed-off-by: Andrew Isherwood <andrew.isherwood@ptfs-europe.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>