Bug 26696: Fix colspan in payment table when credit exists

Bug 26696: Fix colspan in payment table when credit exists

The colspan was adjusted for the total due when a new column
was added to the table, but the 2 lines for credits were missed.

To test:
- Go to any patron account
- Create a manual invoice
- Create a credit
- Go to the 'Make a payment' tab
- Veriy that the total lines at the bottom of the table are
  slightly off
- Apply the patch
- Verify that the total lines in the table now display correctly

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>