Bug 27898: Make PUT /holds/:hold_id accept overrides

Bug 27898: Make PUT /holds/:hold_id accept overrides

This patch makes the route handle overriding pickup locations by passing
the 'x-koha-override' header (set to 'any').

To test:
1. Apply the unit tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/holds.t
=> FAIL: Tests fail, because things cannot be overridden ATM.
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: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>