nnrss should borrow nnshibmun's RSS date processor...or something

[Mark Plaksin <happy@xxxxxxxxxxxxx>]

Some RSS feeds provide the date in ISO 8601 date format.  sb-rss.el from
nnshimbun converts from ISO 8601 to a format that Gnus can handle.  nnrss
does no conversion so Gnus ends up saying the date is the start of the Unix
epoch.

It would be nice if the conversion function from sb-rss.el were put into
some library that nnrss could easily call.  It's not clear to me where the
best place for that would be.  For the moment, I hacked my copy of nnrss.el
to call an un-shimbuned version of shimbun-rss-process-date.