message: http3-server: Fix transfer test
  Remember number of bytes already sent to the client.
diff: @@ -106,7 +106,7 @@ impl Http09Server {
  }
  Some(conn_state) => {
  conn_state.writable = true;
  - if let Some((data, mut offset)) = &mut conn_state.data_to_send {
  + if let Some((data, mut offset)) = &conn_state.data_to_send {
  let sent = conn
  .borrow_mut()
  .stream_send(stream_id, &data[offset..])
  @@ -119,6 +119,7 @@ impl Http09Server {
  self.conn_state.remove(&(conn.clone(), stream_id));
  } else {
  conn_state.writable = false;
  + conn_state.data_to_send = Some((data.to_vec(), offset));
  }
  }
  }
  