Check isConnected in connect() method to avoid duplicate connections

[allaire]

I think it would make sense to make sure we are not already connected in the connect() method, because if you call connect to an active connection, it breaks it.

[allaire]

Finally wrapped it in my manager class, but I think it would make sense to guard the connect method with isConnected. I don't see much value in having multiple connections under the same ActionCableClient instance, or maybe I'm missing something?

A similar idea is present in the ActionCable JS client: https://github.com/rails/rails/blob/master/actioncable/app/assets/javascripts/action_cable/connection.coffee#L24