Client-ref in delivery receipts

[boenrobot]

According to the reference docs at: https://developer.nexmo.com/api/sms#delivery-receipt

The delivery receipt should feature a client-ref if one was supplied at the time the SMS was sent.

There is no getter for it on the DeliveryReceipt object though. The example on the site doesn't feature it either, but that can be forgiven, since whether client-ref is present depends on whether one was sent.