emacs: use 'gnus-decoded in notmuch-mm-display-part-inline ()

When mail message is read from emacs, the message structure
obtained may contain parts which have content included
(`text/plain` for example) and other parts where content is not
included (`text/html` for example).

In case content is included, the string is already available in
emacs' internal format and therefore mm-... functions should not
attempt to do further decoding for the data in temp buffer
provided for it.

Currently when reply buffer is created,
notmuch-mm-display-part-inline () is used to provided quoted reply
content. This change makes the mm-... functions called by it use
'gnus-decoded as charset whenever the content is already available.

File .../emacs-23.3/lisp/gnus/mm-uu.el mentions:
"`gnus-decoded' is a fake charset, which means no further decoding."