handle base64 encoded dn attr

DN can be base64 encoded.

RFC 2849:

    base64-distinguishedName = BASE64-UTF8-STRING
                               ; a distinguishedName which has been base64
                               ; encoded (see note 10, below)
Assuming `unpack('m')` is used here because it's also used below? Should be using `Base64.decode64` but that can be polished separately, too.