[FIX] stock_landed_costs: How change split method default value

Steps to reproduce:

- Create a default value = 'by_volume' for the field Split method on model stock.landed.cost.lines
- Create a landed cost and add a line

Bug:

The default value was always 'equal'.

opw:2161334

closes odoo/odoo#42460

X-original-commit: 34d93802f33dd845c5fce8a3838c98db18bd7484
Signed-off-by: Simon Goffin (sig) <sig@openerp.com>

addons/stock_landed_costs/models/stock_landed_cost.py

@@ -292,7 +292,7 @@ class LandedCostLine(models.Model):
         if not self.product_id:
             self.quantity = 0.0
         self.name = self.product_id.name or ''
-        self.split_method = 'equal'
+        self.split_method = self.split_method or 'equal'
         self.price_unit = self.product_id.standard_price or 0.0
         accounts_data = self.product_id.product_tmpl_id.get_product_accounts()
         self.account_id = accounts_data['stock_input']