public boolean setIfAbsentAndValid(String headerName, String value) {
        Objects.requireNonNull(value, "value");
        Objects.requireNonNull(headerName, "headerName");
        if (isValid(headerName) && isValid(value)) {
            String normalName = HeaderName.normalize(headerName);
            return setIfAbsentNormal(headerName, normalName, value);
        }
        return false;
    }
Validates and adds the name and value to the headers, except if the name is already present.  Unlike
{@link #set(String, String)}, this method does not accept a {@code null} value.

@return if the value was successfully added.
zuul-core/src/main/java/com/netflix/zuul/message/Headers.java