Bug 28370: Add missing additionalProperties in spec

Bug 28370: Add missing additionalProperties in spec

This patch adds missing attribute to the object definitions in the spec.
We wrongly supposed it was the default, but it proved not.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1
=> SUCCESS: Tests pass, nothing breaks!
3. 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: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>