September Gnus provides separation of gnus-original-article-buffer
and gnus-article-buffer, and tm-sgnus supports automatic MIME preview
feature to use these separated article buffers. For MIME article,
gnus-article-buffer indicates MIME processed preview buffer, and
gnus-original-article-buffer indicates raw message.

  gnus-summary-save-in-* save gnus-article-buffer. I think is is not
good. Because I want to MIME navigate again for saved article. So I
think gnus-summary-save-in-* should save gnus-original-article-buffer
instead of gnus-article-buffer. (However for
gnus-summary-save-in-file, gnus-article-buffer may be better)

  So I made a following patch. What do you think about?