Check if channel is open before sending data to avoid errors "InvalidStateError: Failed to execute 'send' on 'RTCDataChannel': RTCDataChannel.readyState is not 'open'" with datachannel or "Janus session was disposed" with websocket when a user leave.
This closes #60
Check if channel is open before sending data to avoid errors "InvalidStateError: Failed to execute 'send' on 'RTCDataChannel': RTCDataChannel.readyState is not 'open'" with datachannel or "Janus session was disposed" with websocket when a user leave. This closes #60