Bug 7606: Editing a vendor in acq creates a new vendor

Bug 7606: Editing a vendor in acq creates a new vendor

To test:

Before patch:
- editing a new vendor will create a new vendor

After patch:
- editing a vendor saves changes correctly, no new vendor is created

Also:
- after saving or cancelling an edit page redirects correctly
- adding new vendors works correctly

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Followed test plan and all tests pass.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
This is a follow-up for 7113