Unsubscribing issue in Broadcast class

[mdi80]

Hi,

I was working with your Broadcast class and noticed a bug related to websocket disconnection. In the subscribe method of the Broadcast class, when a user disconnects from the websocket, the code after yield Subscriber(queue) will never execute. This could potentially lead to increased memory usage if items are never removed from the _subscribers dictionary.

Thank you for your great code!