Bug 23843: Add mapping to Koha::Acquisition::Bookseller

Bug 23843: Add mapping to Koha::Acquisition::Bookseller

This patch adds a to_api_mapping method to the class. This in effect
enables calling ->to_api on the object. The mapping is borrowed from the
API controller. It is not removed from the controller so we are able to
verify (through the tests) that there is no behavior change.
Once this is pushed we need to implement the counter-wise methods and
clean the controllers.
To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/acquisitions_vendors.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat (1)
=> SUCCESS: Tests still pass!
4. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>