Bug 18731: Overload K::A::Fund->to_api to avoid conflict

Bug 18731: Overload K::A::Fund->to_api to avoid conflict

This patch overloads the to_api methods on the Fund class, so
conflicting (on mapping) attribute names are not a problem.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Acquisition/Fund.t \
           t/db_dependent/Koha/REST/Plugin/Objects.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>