'disconnect' event is not emitted on cluster.worker #1304

Demo app here: https://gist.github.com/Olegas/c2e5550911cbe8b96dae

Steps to reproduce:

1. Start app, wait 10 sec
2. App is terminated and log show the following lines

Master started
Worker started
Worker listening
Disconnecting worker
Cluster: worker is disconnected

Expected result:
Line Worker disconnected (from https://gist.github.com/Olegas/c2e5550911cbe8b96dae#file-demo-js-L26) is present in log as stated in docs: https://iojs.org/api/cluster.html#cluster_event_disconnect_1

If I change line 25 to cluster.worker.process.on('disconnect', ... it works fine.