Bug 18942: Internal Server Error symptomatic of using system user

Bug 18942: Internal Server Error symptomatic of using system user

TEST PLAN
---------
1) sudo koha-shell -c bash kohadev
2) prove -v t/Budgets/CanUserUseBudget.t
   -- works
3) prove -v t/Budgets/CanUserModifyBudget.t
   -- works
4) exit
5) git bz apply 18942
   -- interactive just the test patch
6) repeat steps 1-4
   -- prove fails (similar message with 919 in comment #0)
7) git bz apply 18942
   -- interactive just the actual fix
8) repeat steps 1-4
   -- works
9) run koha qa test tools

Signed-off-by: Lee Jamison <ldjamison@marywood.edu>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>