Fix changing the pizza order status using PATCH

Ghost User
2018-02-09T14:33:16+01:00

Closes #567

#### Previous behaviour
Steps to reproduce:

1. Create a pizza event
2. Add orders
3. Change the order status in the pizza frontend admin
4. The user connected to the order changes to _you_, the logged-in user. And the paid status changes.

#### New behaviour
Steps to validate that it works:

1. Create a pizza event
2. Add orders
3. Change the order status in the pizza frontend admin
4. The user connected stays the same. The paid status changes.