* gnus-art.el (gnus-mime-display-single): Use utf-8 by default for decoding

* gnus-art.el (gnus-mime-display-single): Use utf-8 by default for decoding
 text/calendar parts.

* message.el (message-forward-make-body-mime): Always mark body as having no
 illegible text; remove signed-or-encrypted argument.
(message-forward-make-body): Don't pass signed-or-encrypted arg to it.

* mml.el (mml-generate-mime): Make sure it uses multibyte temp buffer.
(mml-generate-mime-1): Don't encode body if it is specified to be in raw form;
 don't make buffer be unibyte when inserting multibyte string.