message: http3-server: Fix transfer test
  Replaces PR826 with a better fix for issue 819
diff: @@ -106,20 +106,19 @@ impl Http09Server {
  }
  Some(conn_state) => {
  conn_state.writable = true;
  - if let Some((data, mut offset)) = &conn_state.data_to_send {
  + if let Some((data, ref mut offset)) = &mut conn_state.data_to_send {
  let sent = conn
  .borrow_mut()
  - .stream_send(stream_id, &data[offset..])
  + .stream_send(stream_id, &data[*offset..])
  .unwrap();
  eprintln!("Wrote {}", sent);
  - offset += sent;
  - if offset == data.len() {
  + *offset += sent;
  + if *offset == data.len() {
  eprintln!("Sent {} on {}, closing", sent, stream_id);
  conn.borrow_mut().stream_close_send(stream_id).unwrap();
  self.conn_state.remove(&(conn.clone(), stream_id));
  } else {
  conn_state.writable = false;
  - conn_state.data_to_send = Some((data.to_vec(), offset));
  }
  }
  }
  