Bug 15898 - Use Koha::Account::pay internally for makepartialpayment

Bug 15898 - Use Koha::Account::pay internally for makepartialpayment

This is the fourth patch in a series to unify all payment functions into
a single mathod

Test Plan:
1) Apply this patch
2) prove t/db_dependent/Accounts.t
3) Test fine payment via the "Pay" button,
   but make the payment for less then the full amount

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>