Bug 31351: (QA follow-up) Use $self->json in Background modules

Bug 31351: (QA follow-up) Use $self->json in Background modules

Making the disabling utf8 flag explicit instead of depending on
the default of the CPAN module.

Incorporating the change in background_jobs_worker too.

Test plan:
See next patches when we look at unit tests.
Restart koha-worker.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>