Bug 25662: Make the route for holds restpect maxreserves

Bug 25662: Make the route for holds restpect maxreserves

This patch fixes the behaviour for the POST /holds route. It assumed
maxreserves was checked in CanItemBeReserved which is not the case.

Tests are added to check for this behaviour.

To test:
1. Apply the regression tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t
=> FAIL: Tests fail!
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>