Bug 20599: Add the Koha::Subscription->vendor method

Bug 20599: Add the Koha::Subscription->vendor method

Test plan:
  prove t/db_dependent/Koha/Subscription.t
must return green

NOTE: My preference is for $subscription to be object,
      and perhaps something like $built_subscription to
      be from the builder. Because, $object is a poor
      variable name.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>