Bug 29957: Let check_api_auth return created cookie

Bug 29957: Let check_api_auth return created cookie

The cookie created on L1496 is useless, since it is not returned.
We could either remove the cookie creation (unchanged behavior).
But since check_api_auth is expected to return a cookie when it is ok,
I opt for returning the cookie here (corrected behavior).

Test plan:
Logout in staff.
Check on staff: /cgi-bin/koha/svc/localization?id=1
You should have a 400 response.
Login with staff credentials (incl. manage_itemtypes)
Revisit same URL.
You should see a JSON response.
Check if you can hit other staff pages.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>