Bug 20942: (QA follow-up) Use $lines->total_outstanding

Bug 20942: (QA follow-up) Use $lines->total_outstanding

This patch makes the controller code use $lines->total_outstanding
instead of expecting ->outstanding_debits and ->outstanding_credits
return the total.

Tests should pass as before. No behaviour change is expected.

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/patrons_accounts.t
=> SUCCESS: Tests still pass, no behaviour change.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>