[fix] fixed validation when register an user

Yes, but I think it's more meant as an example. This extension is used in the integration-tests, but it should not defined here...? It also has to be [configured](https://github.com/osiam/connector4java-integration-tests/blob/master/src/test/resources/addon-self-administration-configuration/addon-self-administration.properties#L32) to be usable. So it should be removed and also in the form template, but has to be documented, how to configure an own extension... and it is [already](https://github.com/osiam/addon-self-administration/wiki/User_registration#orgosiamhtmlformextensions) :dancer: