[16.0] grouping doesn't take into account the mandate

[alexis-via]

I found this bug while reading the code of OCA/bank-payment... then I tested the corresponding scenario and it confirmed the bug.

Here is the scenario:

• create a new customer with 1 bank account. Set customer payment mode to SEPA direct debit. Create 2 mandates linked to that bank account.
• create a customer invoice for that new customer with mandate 1.
• create a customer invoice for that new customer with mandate 2.
• add the 2 customer invoices to a new direct debit order. Confirm that debit order: you will see a single account.payment, instead of 2. When you generate the XML file, you will have only 1 transaction linked to only 1 of the 2 mandates.