* gnus-art.el (gnus-default-article-saver): Add gnus-summary-save-in-pipe to

* gnus-art.el (gnus-default-article-saver): Add gnus-summary-save-in-pipe to
 choices.
(gnus-summary-save-in-pipe): Add :decode and :headers properties;
 use gnus-summary-pipe-output-default-command as the default command.
(gnus-summary-pipe-to-muttprint): Update gnus-summary-muttprint-program instead
 of gnus-last-shell-command.

* gnus-sum.el (gnus-summary-pipe-output-default-command): New user option.
(gnus-summary-muttprint-program): Mention the value will be changed.
(gnus-summary-save-article): Force showing of all headers.
(gnus-summary-pipe-output): Work with the 2nd argument HEADERS.