When invoicing from dropshipping, the same picking serves for both
invoicing sale and purchase orders. The OCA module
stock_dropshipping_dual_invoice makes that this happens, but
as the picking is the same, it contains both the sale and the
purchase reference, making that the resulting invoices don't have
the correct payment mode. This is harmless for the rest of
operations, as the dictionary query is provided with the proper
default value. On v9, dropshipping dual invoice has been merged
into core, so this patch for sure will be welcome in that version
(if the used method is the same).
