Bug 16895 - Allow writeoffs via SIP2

Bug 16895 - Allow writeoffs via SIP2

Many SIP2 services such as those by Comprise Technologies are able to or
require that an ILS be able to accept writeoffs via SIP2. The SIP2
protocol specifies that payment type be a two digit number, but does not
specify a code for writeoffs. To this end we should allow the write-off
code to be specified in the SIP2 config on a per-account basis so that
if different vendors use different fixed codes for write-offs we can
handle that gracefully.

Test Plan:
1) Apply this patch
2) Modify your SIP2 config to include
      payment_type_writeoff="06"
  in the login portion of the account you will be using for the test.
3) Restart your SIP2 server
4) Create a fee for a patron
5) Send a SIP2 fee paid message specifying the payment type code we
   defined earlier, with a payment amount that is *not* equal to the
   amount outstanding for the fee.
6) Note the fee paid response indicates the payment failed
7) Repeat step 5, but this time send the amount outstanding as the
   payment amount
8) Note that the fee paid response indicates a successful payment
9) Note in Koha that the fee has been written off!

Signed-off-by: Rhonda Kuiper <kuiper@roundrocktexas.gov>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>