Line 191 -195, it is trying to find value for header "Proxy-Authentication". This is a typo. It should be "Proxy-Authenticate"

        boolean proxy = status == PROXY_UNAUTHORIZED; //Actually 407
        String authname = proxy ? "Proxy-Authentication" : "WWW-Authenticate";
        String authval = hdrs.firstValue(authname).orElseThrow(() -> {
            return new IOException("Invalid auth header");
        });

Please refer to the RFC https://tools.ietf.org/html/rfc2617#section-3.6
As documented:
   Upon receiving a request which requires authentication, the proxy/server
   must issue the "407 Proxy Authentication Required" response with a
   "Proxy-Authenticate" header.