Impacted versions:
* 11.0

Steps to reproduce:
* Create company A
* Create company B
* Admin user with both companies but currently in company B
* Create Demo user only with company B
* Create fiscal position A to company A
* Create fiscal position B to company B
* Create vendor with fiscal position A with company A and the same vendor with fiscal position B in company B
* Create product and add the vendor created before
* Set like **Create a draft purchase order**
* Logging with Demo user
* Create sale order with one sale order line with route **make to order**
* Validate sale order

Current behavior:
The purchase order created by sale order has the fiscal position **company A**

Expected behavior:
The purchase order must have fiscal position **company B**
addons/purchase/models/purchase.py
'date': self.date_order,
            'origin': self.name,
            'location_dest_id': self._get_destination_location(),
            'location_id': self.partner_id.property_stock_supplier.id,
            'company_id': self.company_id.id,
        }
    @api.multi
    def _create_picking(self):
        StockPicking = self.env['stock.picking']
        for order in self:
            if any([ptype in ['product', 'consu'] for ptype in order.order_line.mapped('product_id.type')]):
                pickings = order.picking_ids.filtered(lambda x: x.state not in ('done','cancel'))
                if not pickings:
                    res = order._prepare_picking()
                    picking = StockPicking.create(res)
                else:
                    picking = pickings[0]
                moves = order.order_line._create_stock_moves(picking)
                moves = moves.filtered(lambda x: x.state not in ('done', 'cancel')).action_confirm()
                seq = 0
                for move in sorted(moves, key=lambda move: move.date_expected):
                    seq += 5
                    move.sequence = seq
                moves.force_assign()
                picking.message_post_with_view('mail.message_origin_link',
                    values={'self': picking, 'origin': order},
                    subtype_id=self.env.ref('mail.mt_note').id)
        return True
    @api.multi
    def _add_supplier_to_product(self):