private var lastModified: Date? = null
    private var lastModifiedString: String? = null

    /**
     * The expiration date of the cached response, if known. If both this field and the max age are
     * set, the max age is preferred.
     */
    private var expires: Date? = null

    /**
     * Extension header set by OkHttp specifying the timestamp when the cached HTTP request was
     * first initiated.
     */