private static boolean isValid(@Nullable String value) {
        if (value == null || findInvalid(value) == ABSENT) {
            return true;
        }
        invalidHeaderCounter.increment();
        return false;
    }
Checks if the given value is compliant with our RFC 7230 based check
zuul-core/src/main/java/com/netflix/zuul/message/Headers.java