[FIX] website_sale: reset sale order from session if status is no more in draft

When a sale order has in state != draft, we raise a error message since commit
f6c65a3d9e.

But if we don't reset the sale order id from the session, the user cannot
continue to navigate on website because the user will have the error each time.

addons/website_sale/models/sale_order.py

@@ -75,6 +75,7 @@ class sale_order(osv.Model):
         quantity = 0
         for so in self.browse(cr, uid, ids, context=context):
             if so.state != 'draft':
+                request.session['sale_order_id'] = None
                 raise osv.except_osv(_('Error!'), _('It is forbidden to modify a sale order which is not in draft status'))
             if line_id != False:
                 line_ids = so._cart_find_product_line(product_id, line_id, context=context, **kwargs)