message: fix(server): fix debug assert failure when kept-alive connections see a parse error
diff: @@ -550,6 +550,9 @@ where I: AsyncRead + AsyncWrite,
  return Err(::Error::new_version_h2())
  }
  if let Some(msg) = T::on_error(&err) {
  + // Drop the cached headers so as to not trigger a debug
  + // assert in `write_head`...
  + self.state.cached_headers.take();
  self.write_head(msg, None);
  self.state.error = Some(err);
  return Ok(());
  