Bug 20980: (follow-up) Only use UpdateStats for 'payment' and 'writeoff'

Bug 20980: (follow-up) Only use UpdateStats for 'payment' and 'writeoff'

This patch makes Koha::Account::add_credit record statistics logs
(through C4::Stats::UpdateStats) only for the 'payment' and 'writeoff'
credit types.

Both credit types are whitelisted in UpdateStats, so we keep the current
behaviour. Another option would've been to add new account types to the
white list, but with the account offsets it seems that using the
statistics table for account changes won't be useful for too long, as
offsets contain the same (and more) information.

To test:
- Apply the patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>