payment: fix NPE in PaymentMethodProcessor

The account may not have a default payment method yet.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
payment: fix test wiring setup

The test plugin name didn't match the value passed in the
addPaymentMethod call. Tests broke after 00dc2dbadd8ec3e2cdd524b9e683f955dac67a8f.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>