Bug 18729: (follow-up) Adjust API to new spec

Bug 18729: (follow-up) Adjust API to new spec

The previous patch introduces some behavioural changes to the API, as
well as the data types that need to be passed; all happens in the tests.

This patch adapts the route so it complies with those changes:
- JSON object containing pickup_library_id attribute is now passed back
  and forth.
- The controller should take care of checking the pickup location is
  valid, using the available tools.

To test:
1. Apply the patches, up to the tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t
=> ERROR: Tests fail because the controller doesn't implement the
desired behavior
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: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>