message: Fix logging of ack_open in stream_msg_visitor
diff: @@ -111,7 +111,7 @@ auto stream_msg_visitor::operator()(stream_msg::ack_open& x) -> result_type {
  self_->current_sender(), d, false),
  i_};
  }
  - CAF_LOG_DEBUG("received stream_msg::ok for unknown stream");
  + CAF_LOG_WARNING("received stream_msg::ack_open for unknown stream");
  return {sec::unexpected_message, e_};
  }
  