Check signature.

        https://devdocs.line.me/en/#webhook-authentication

        :param str body: Request body (as text)
        :param str signature: X-Line-Signature value (as text)
        :rtype: bool
        :return: result

def validate(self, body, signature):
        """Check signature.

        https://devdocs.line.me/en/#webhook-authentication

        :param str body: Request body (as text)
        :param str signature: X-Line-Signature value (as text)
        :rtype: bool
        :return: result
        """
        gen_signature = hmac.new(
            self.channel_secret,
            body.encode('utf-8'),
            hashlib.sha256
        ).digest()

        return compare_digest(
                signature.encode('utf-8'), base64.b64encode(gen_signature)
        )