message: Fix serialization of ack_batch
diff: @@ -173,7 +173,7 @@ template <class Inspector>
  typename Inspector::result_type inspect(Inspector& f,
  upstream_msg::ack_batch& x) {
  return f(meta::type_name("ack_batch"), x.new_capacity, x.desired_batch_size,
  - x.acknowledged_id);
  + x.acknowledged_id, x.max_capacity);
  }
  /// @relates upstream_msg::drop
  