public synchronized Service trustForwardHeaders() {
        if (initialized) {
            throwBeforeRouteMappingException();
        }
        this.trustForwardHeaders = true;

        return this;
    }
Sets Spark to trust the HTTP headers that are commonly used in reverse proxies.
More info at https://www.eclipse.org/jetty/javadoc/current/org/eclipse/jetty/server/ForwardedRequestCustomizer.html
src/main/java/spark/Service.java