Bug 21969: Fix Koha::Account->outstanding_* matching the opposite on pathological...

Bug 21969: Fix Koha::Account->outstanding_* matching the opposite on pathological cases

This patch makes outstanding_* methods be safe regarding pathological
account lines that get converted into another type because of the value
of amountoutstanding

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> FAIL: Tests fail because pathological account lines are wrongly
picked.
- Apply this patch
- Run:
 k$ prove t/db_dependent/Koha/Account.t
=>SUCCESS: All green!

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>