Creating channel later on does not work

First: great work. Thanks!

When I create a channel after using an existing one, the subscribe is not called.

I just looked into the code and found a way how to solve this issue

If I call `onopen` immediately an not on next tick (what is strangely not working) it works like a charm.

https://github.com/sockjs/websocket-multiplex/blob/master/multiplex_client.js#L68

Whats the problem here?
