[FIX] stock, sale_stock: copy the to_refund field when splitting a move

The to_refund attribute is set to copy=False.
However when splitting a move to create a backorder, a new move is created as
a copy with the remaining work to do.
This attribute is needed so that the backorder refund is consistently applied.

Essentially a backport of commit:
https://github.com/odoo/odoo/commit/c5ffca151160f9d99bec036330def108e4e91e0b

opw 1865324

addons/sale_stock/models/stock.py

@@ -40,6 +40,11 @@ class StockMove(models.Model):
                     subtype_id=self.env.ref('mail.mt_note').id)
         return result
 
+    def _prepare_move_split_vals(self, defaults):
+        defaults = super(StockMove, self)._prepare_move_split_vals(defaults)
+        defaults['to_refund_so'] = self.to_refund_so
+        return defaults
+
 
 class StockPicking(models.Model):
     _inherit = 'stock.picking'

addons/stock/models/stock_move.py

@@ -938,6 +938,10 @@ class StockMove(models.Model):
             new_move_prop = self.move_dest_id.split(qty)
             new_move.write({'move_dest_id': new_move_prop})
 
+    def _prepare_move_split_vals(self, defaults):
+        # hook to add values in default vals in other modules
+        return defaults
+
     @api.multi
     def split(self, qty, restrict_lot_id=False, restrict_partner_id=False):
         """ Splits qty from move move into a new move
@@ -967,6 +971,9 @@ class StockMove(models.Model):
             'move_dest_id': self.move_dest_id.id,
             'origin_returned_move_id': self.origin_returned_move_id.id,
         }
+
+        defaults = self._prepare_move_split_vals(defaults)
+
         if restrict_partner_id:
             defaults['restrict_partner_id'] = restrict_partner_id