Bug 20167: Make changing a hold pass the itemnumber to ModReserve

Bug 20167: Make changing a hold pass the itemnumber to ModReserve

If you have an item-level hold, changing an attribute (like priority,
pickup location or suspended until date) makes the hold a biblio-level
hold, because ModReserve is not passed the current itemnumber.

This patch uses the hold's itemnumber and passes it to the ModReserve
call.

To test:
- Run the regression tests from the previous patch:
  $ kshell
 k$ prove t/db_dependent/api/v1/holds.t
=> FAIL: Tests fail
- Apply this patch
- Run the tests:
  $ kshell
 k$ prove t/db_dependent/api/v1/holds.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>