docs: private Request createTunnelRequest() throws IOException {
      Request proxyConnectRequest = new Request.Builder()
          .url(route.address().url())
          .method("CONNECT", null)
          .header("Host", Util.hostHeader(route.address().url(), true))
          .header("Proxy-Connection", "Keep-Alive") // For HTTP/1.0 proxies like Squid.
          .header("User-Agent", Version.userAgent)
          .build();
  
      Response fakeAuthChallengeResponse = new Response.Builder()
          .request(proxyConnectRequest)
          .protocol(Protocol.HTTP_1_1)
          .code(HttpURLConnection.HTTP_PROXY_AUTH)
          .message("Preemptive Authenticate")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(-1L)
          .header("Proxy-Authenticate", "OkHttp-Preemptive")
          .build();
  
      Request authenticatedRequest = route.address().proxyAuthenticator()
          .authenticate(route, fakeAuthChallengeResponse);
  
      return authenticatedRequest != null
          ? authenticatedRequest
          : proxyConnectRequest;
    }