[FIX] account_payment_group: method support multiple records
[FIX] account_payment_group: method support multiple records

Using self.field name throws singleton error. iterating in the
records solves the problem

closes ingadhoc/account-payment#305

Signed-off-by: Katherine Zaoral <efb215f8f79524f9503c2254db6f4fe97383655c@adhoc.com.ar>

ingadhoc/account-payment