subscribe method
Implementation
Socket subscribe(String channel, [AckCall? ack]) {
int count = ++_counter;
var message = {
'event': '#subscribe',
'data': {'channel': channel},
'cid': count
};
if (ack != null) _acks[count] = getAckObject(channel, ack);
var json = jsonEncode(message);
sendOrAdd(json);
return this;
}