This patch simplifies the payload as suggested on bug 25855. It also
keeps some specific params that cannot be deduced from the passed
checkout object, (e.g. if it is an onsite checkout).
Tests are cleared and added for this special exceptions.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>