[FIX] purchase: specific search alter order duplication

1) Go to purchasing, and in the search area type "8888"
2) Click on the little arrow next to "Search Product for: 8888"
3) Select "[FURN_8888] Office Lamp"
4) Choose Purchase Order PO00009 which has FURN_8888 on the second line:
5) Choose Action > Duplicate

The 1st line end up with the searched product, not the original product.
This happens because the search add the context key
'default_product_id'. By chance the product.product model has a
'product_id' field which may happen to be not filled.
This cause the model api to look for a default value in the context and the
first line of the order get overridden.

Removing the specific key from the env solve the issue

opw-2188429

closes odoo/odoo#44598

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/purchase/models/purchase.py

@@ -168,6 +168,9 @@ class PurchaseOrder(models.Model):
 
     @api.multi
     def copy(self, default=None):
+        ctx = dict(self.env.context)
+        ctx.pop('default_product_id', None)
+        self = self.with_context(ctx)
         new_po = super(PurchaseOrder, self).copy(default=default)
         for line in new_po.order_line:
             seller = line.product_id._select_seller(