private static String validateField(@Nullable String value) {
        if (value != null) {
            int pos = findInvalid(value);
            if (pos != ABSENT) {
                invalidHeaderCounter.increment();
                throw new ZuulException("Invalid header field: char " + (int) value.charAt(pos) + " in string " + value
                        + " does not comply with RFC 7230");
            }
        }
        return value;
    }
Checks if the input value is compliant with our RFC 7230 based check
Returns input value if valid, raises ZuulException otherwise
zuul-core/src/main/java/com/netflix/zuul/message/Headers.java