[FIX] mrp: missing cast string to date

Usecase to repdroduce:
- Create a manufactured product with route manufacture and a RR
- Run the scheduler.

Orderpoint will prepare procurement values's date as string. Other
module could possibly also use string as date and a call to from_string
will not break anything if the value is already a date or a datetime.

closes odoo/odoo#38895

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>

addons/mrp/models/stock_rule.py

@@ -84,7 +84,7 @@ class StockRule(models.Model):
             'bom_id': bom.id,
             'date_deadline': fields.Datetime.to_string(self._get_date_planned(product_id, company_id, values)),
             'date_planned_finished': values['date_planned'],
-            'date_planned_start': values['date_planned'] - relativedelta(hours=1),
+            'date_planned_start': fields.Datetime.from_string(values['date_planned']) - relativedelta(hours=1),
             'procurement_group_id': False,
             'propagate_cancel': self.propagate_cancel,
             'propagate_date': self.propagate_date,