Allow invoice shipment cost when unit_price is none

The sale_shipment_cost module, when has not price, set quantity to 0 and not invoice the line.

So the client sees that the transport has not been invoiced (missing the shipment concept).

The proposal is allow to do configurable and is from The quantity should not be reset when a sale line is set with unit price 0

  1. Allow configuration section

Model: sale.configuration

sale_shipment_cost_invoice: Selection field with options:
- None
- Always

  1. check configuration to set the quantity when get invoice line quantity:
1 if config.sale_shipment_cost_invoice == 'always' else 0

What is the point to open a feature topic when there is already a bug report?

discuss how to do? configurable? is a bug?

I do not think we should make it configurable but just support that the line is created when the cost is empty.

Aslo we should take in account that the system may compute a cost of 0 in the sale, but update the cost latter in the shipment. So the final cost is what should be invoiced to the customer.

P.S: You can discuss it directly on the issue, so all the discussion is in a single place.