Bug 27636: Update ->pay to use ->payin_amount internally

Bug 27636: Update ->pay to use ->payin_amount internally

This patch is an interim patch to update Koha::Account->pay to use the
new Koha::Account->payin_amount method internally. It simply utilises
that method for the logic and acts as an input/output shim for
compatability and to allow us to use the existing tests.

Test plan
1/ Run t/db_dependent/Koha/Account.t

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>