Bug 29759: Make cancelling an AR refund

Bug 29759: Make cancelling an AR refund

This patch makes the Koha::ArticleRequest->cancel method perform a
refund if it applies.

The sequence is:
- Find refundable (potentially partial) payments against the fee
- Generate a refund for the refundable amount
- Reduce the fee to zero
=> RESULT: the patron owes nothing, any credits applied to the debit are
refunded.

1. Apply the unit tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/ArticleRequest.t
=> FAIL: Nothing is refunded
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass! Refunds take place!
5. Try on the UI
=> SUCCESS: All good
6. Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>