public static void addXForwardedHeader(Headers headers, HeaderName name, String latestValue) {
        if (OVERWRITE_XF_HEADERS.get()) {
            headers.set(name, latestValue);
        } else {
            // If this proxy header already exists (possibly due to an upstream ELB or reverse proxy
            // setting it) then keep that value.
            String existingValue = headers.getFirst(name);
            if (existingValue == null) {
                // Otherwise set new value.
                if (latestValue != null) {
                    headers.set(name, latestValue);
                }
            }
        }
    }
User: michaels@netflix.com
Date: 6/8/15
Time: 11:50 AM
zuul-core/src/main/java/com/netflix/zuul/util/ProxyUtils.java