Don't re-compress .gz & al. in notmuch-show-save-part.

Don't re-compress .gz & al. in notmuch-show-save-part.

Write-region handles some file names specially, see Emacs Lisp
manual section 25.11 Making Certain File Names "Magic" [1].  This
is a nice feature for normal text editing, but it is not
desirable if we need to save raw file content (e.g. attachment).
In particular, this affects archives and may result in corrupted
attachments saved with notmuch-show-save-part (attachment button
click handler).

Turns out, smart GNUS folks encountered the same problem and
implemented write-region wrapper which inhibits some file name
handlers.  In particular, this wrapper is used in mm-save-part,
which is why notmuch-save-attachments that uses it works fine
with archives.

The patch replaces write-region with mm-write-region in
notmuch-show-save-part.  Also it removes coding-system-for-write
and require-final-newline setting in notmuch-show-save-part.  The
former is set in mm-write-region.  The latter seems to be
unneeded because mm-save-part does not use it.

[1] http://www.gnu.org/s/emacs/manual/html_node/elisp/Magic-File-Names.html