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)
Yes https://github.com/mattias-ohlsson/ruby-net-ldap/commit/223797095c519d18248526af2a184d223c79d8b7