Motivation:
An implementation of the `FilteredStreamMessage` might have resources to release when
the `StreamMessage` is complete. For example, an `HttpResponse` from `ContentPreviewingService` should call
`ContentPreviewer.produce()` to release the resource and produce the content preview.
However, the `FilteredStreamMessage` is not notified when the `StreamMessage` is canceled by the `Subscriber`.

Modification:
- Add `FilteredStreamMessage.beforeCancel()` so that an implementation of the message
  can add a hook before `Subscription.cancel()`.

Result:
- You no longer see that the log is not complete when applying `ContentPreviewingService`.