Set subscribed flag to true when subscribing to channel

When subscribing to channel, pusher sets the `subscribed` flag to `true`.

This PR adds the same to the test stub.

![screen shot 2014-11-26 at 5 06 41 pm](https://cloud.githubusercontent.com/assets/3772828/5198831/ef60d5ca-7590-11e4-8864-84f4b6cfeec6.png)