id_order_invoice not set for CDiscount fees

[ghost]

Expected behaviour

When CDiscount fees are set for an order, the corresponding virtual product should appears on the order details and in the order invoice.

Actual behaviour

CDiscount fees are visible only in the order details but not in the invoice.
This is due to id_order_invoice set at 0 when inserting FDG-ShoppingFlux.

Steps to reproduce the behaviour

Create an order with fees

[ghost]

Example of fix to be tested:

We retrieve $idOrderInvoice before inserting the FDG.

            $idOrderInvoice = Db::getInstance()->getValue('
            SELECT `id_order_invoice`
            FROM `'._DB_PREFIX_.'order_invoice`
            WHERE `id_order` =  '.(int)$id_order);

            $fdgInsertFields = array(
                'id_order' => (int) $id_order,
                'id_order_invoice' => empty($idOrderInvoice) ? 0 : (int)$idOrderInvoice,