public String getFirst(String headerName, String defaultValue) {
        Objects.requireNonNull(defaultValue, "defaultValue");
        String value = getFirst(headerName);
        if (value != null) {
            return value;
        }
        return defaultValue;
    }
Get the first value found for this key even if there are multiple. If none, then
return the specified defaultValue.
zuul-core/src/main/java/com/netflix/zuul/message/Headers.java