Bug 27760: Make authenticate_api_request call stash_overrides

Bug 27760: Make authenticate_api_request call stash_overrides

By making this method call the helper, we make sure the overrides
hashref is available to any controller to make us of it.

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/auth_authenticate_api_request.t \
           t/Koha/REST/Plugin/Query.t
=> SUCCESS: Tests pass! The helper works as expected and the
authenticate_api_request method invokes it!
3. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>