Use CouchDocument from new backend (#4475).
Ignore encoding, use message.as_string directly.

The message is already in str type, so we don't care about
encoding. json.dumps will ignore convertion.
Merge remote-tracking branch 'chiiph/bug/multiple_encoding' into develop
Merge remote-tracking branch 'refs/remotes/drebs/bug/4475_use-couch-document' into develop

Conflicts:
	src/leap/mx/mail_receiver.py
Update requirements
Fold in changes
Merge branch 'release-0.3.6'