HTTP Heartbeat

Provides a heartbeat every 10 seconds -- configurable using the
`subscribeHeartbeatDuration` flag -- if no other data has been sent.
This behavior is provided to keep Heroku client connections alive.

The heartbeat is a null byte chunk, e.g. '1\r\n\0\r\n'.

When regular chunks are sent, the heartbeat timer is reset to 10
seconds.