message: transport: If we closed the connection, never send anything
diff: @@ -1580,6 +1580,12 @@ impl Connection {
  self.generators.push(Box::new(CloseGenerator {}));
  self.flow_mgr.borrow_mut().set_need_close_frame(true);
  }
  + State::Closed(..) => {
  + // Equivalent to spec's "draining" state -- never send anything.
  + self.send_streams.clear();
  + self.recv_streams.clear();
  + self.generators.clear();
  + }
  _ => {}
  }
  }
  