Bug 23858: Make POST add the Location header on vendors endpoint

Bug 23858: Make POST add the Location header on vendors endpoint

According to the coding guidelines, this endpoint should be adding the
Location header on POST actions.

To test:
1. Apply the tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/acquisitions_vendors.t
=> FAIL: Tests fail!
3. Apply this patch
4. Repeat (1)
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>