When Jetty's `HttpClient` make a request to a server through a forward proxy, the server may...

When Jetty's HttpClient make a request to a server through a forward proxy, the server may reply with a 407 (Proxy Authentication Required).

The client mistakenly interprets the 407 as coming from the proxy, so it re-sends the request with the Proxy-Authentication header, which arrives to the server, that now has access to the proxy credentials.