private static int findInvalid(String value) {
        for (int i = 0; i < value.length(); i++) {
            char c = value.charAt(i);
            // ASCII non-control characters, per RFC 7230 but slightly more lenient
            if (c < 31 || c == 127) {
                return i;
            }
        }
        return ABSENT;
    }
Validated the input value based on RFC 7230 but more lenient.
Currently, only ASCII control characters are considered invalid.

Returns the index of first invalid character. Returns {@link #ABSENT} if absent.
zuul-core/src/main/java/com/netflix/zuul/message/Headers.java