Bug 14931: (Follow-up) Validate date entry in contracts

Bug 14931: (Follow-up) Validate date entry in contracts

 Go to Home > Acquisitions, search for an active vendor and view the vendor
- Click on 'Contracts' at the left
  (admin/aqcontract.pl)
- Create and edit contracts and verify that it works OK.
  (Note: there is no help for this page, see Bug 14929)

Amended for comment #7  25.10.2015 / mv

Signed-off-by: Jesse Weaver <pianohacker@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>