Bug 9302: (QA follow-up) Consistency follow-up

Bug 9302: (QA follow-up) Consistency follow-up

This patch moves the Koha::Patrons->merge method into
Koha::Patron->merge_with in the line of the discussed implementation for
bug 15336. I agree with that implementation so I provide this follow-up.

Tests are adjusted, the controller script is adapted too. The behaviour
remains.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>