The change I'd made a while ago to gnus-mime-copy-part for jka-compr
auto-decompression never actually worked with emacs 20, it bombs
prompting for a coding system and then sending the wrong bytes out to
gzip.  Eg. press "c" on the foo.txt.gz below to see it happen.

I think the part contents have to be sent out from a unibyte buffer to
avoid conversions,

	* gnus-art.el (gnus-mime-jka-compr-maybe-uncompress): Take contents
	parameter, use a unibyte buffer for that data to get the right bytes
	out of call-process-region under emacs 20.
	(gnus-mime-copy-part): Pass contents parameter.