[FIX] purchase: use order date to set the picking date

When creating a pickign from a purchase order, do not use the schedule date
('date_planned') to specify the creation date ('date') of a picking.
There is already field called Scheduled Date ('min_date') and Max Expected Date
('max_date') that can display this information, this is redundand.

Fixes #4352

addons/purchase/purchase.py

@@ -842,7 +842,7 @@ class purchase_order(osv.osv):
             picking_vals = {
                 'picking_type_id': order.picking_type_id.id,
                 'partner_id': order.partner_id.id,
-                'date': max([l.date_planned for l in order.order_line]),
+                'date': order.date_order,
                 'origin': order.name
             }
             picking_id = self.pool.get('stock.picking').create(cr, uid, picking_vals, context=context)