Bug 21909: Make Koha::Account::outstanding_* preserve context

Bug 21909: Make Koha::Account::outstanding_* preserve context

This patch modifies the methods so they preserve the caller's context
(list and scalar).

To test:
- Apply the first patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> FAIL: outstanding_credits and outstanding_debits tests fail
- Apply this patch
- Run:
 k$ prove t/db_dependent/Koha/Account.t
=> SUCCESS: We are so happy now :-D
- Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>