A hidden field name FORM_TYPE is treated in a special way, as specified in https://xmpp.org/extensions/xep-0068.html#usecases-correct.

Atm slixmpp doesn't do anything special about it and it's up to the user to know to check it/ignore it when looping over fields etc.

I would like us to propose an interface to retrieve/set it, and exclude a correct usage of it from .get_values results. Probably breaking.