Editing UTF-8 encoded mail messages does not seem to work reliably.
I'll append such a message mailed to myself using lynx with "wide"
ncurses support.

The problem: Press e (gnus-summary-edit-article), then C-c C-c.  Saving
back the message Gnus turns it into a base64 encoding and worse, it
does not seem to preserve the UTF-8 charset.