[NoteMessageWrapper] Use unicode also for main body

Summary:
Otherwise `KMime::Message` will think it's `latin1` and mess unicode characters.
This patch restores unicode support in KJots, which was broken by {D29217}.
(not sure if there are other users of `NoteMessageWrapper` now - at least `KNotes` doesn't use it)

Test Plan: Notes with unicode characters now work

Reviewers: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29761