Force registration when using xep_0077 plugin

[botheis]

The registration is not applyed when we use xep_0077 plugin

################ Please use Gitlab instead of Github ###################################

Hello, thank you for contributing to slixmpp!

You’re about to open a pull request on github. However this github repository is not the official place for contributions on slixmpp.

Please open your merge request on https://lab.louiz.org/poezio/slixmpp/

You should be able to log in there with your github credentials, clone the slixmpp repository in your namespace, push your existing pull request into a new branch, and then open a merge request with one click, within 3 minutes.

This will help us review your contribution, avoid spreading things everywhere and it will even run the tests automatically with your changes.

Thank you.