Unsubscribe properly for the disconnection of WebSocket

[as10896]

The exception raised by WebSocket disconnection fails the unsubscribing behavior of the Subscriber context manager. Fixed it by moving the unsubscribing snippet to the finally block.

[andrew222651]

The try should maybe be moved down to before the yield.

[alex-oleshkevich]

Fixed in https://github.com/encode/broadcaster/pull/112