[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: [KJots] Port to NoteMessageWrapper.
(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