Hi!

Once in a while I receive an email that Gnus fails to show, giving an
error in rfc2231-decode-encoded-string.  As far as I could see, the
problem is with the email message, that has an incorrectly formed
RFC2231 string somewhere within it.  More specifically, the string ends
with '%'.  Probably, this is due to some broken MUA out there.

Still, I think that Gnus should handle this case more gracefully, rather
than not showing the email at all.

Therefore, I propose the following change to the
rfc2231-decode-encoded-string to make it more robust.