Bug 20980: (follow-up) Offset name consistency

Bug 20980: (follow-up) Offset name consistency

This patch makes 'credit' account types generate 'Manual Credit' offset.
The original patch used 'Payment' but I think the code wants to
distinguish between payments and manually added credits (otherwise they
would've picked the same accounttype value, and they don't.

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>