RSS feeds: fix timestamp

[Reported by][0] /u/josh64.

When the message RSS feeds were created, they erroneously had an extra timezone
appended to the `dc:date` element, which causes validation errors.

In most of the feeds we rely only upon `pubDate`; it's not clear to me why we
additionally use `dc:date` in the few places we do, but I suppose it doesn't
hurt anything.  The other two places using it are `subreddit.xml`, which never
had the extra timezone, and `comment.xml`, in which it was removed three years
ago (reddit/reddit@5675398).  This fixes the last incorrect instance.

Note: Although the reporter says they got an error in the W3C validator and the
format is clearly wrong, in my tests the validator passed the previous,
incorrect feed. *shrug*

[0]: http://redd.it/2r1plf

r2/r2/templates/message.xml

@@ -54,7 +54,7 @@
   ${thing.subject} : ${unsafe(taglinetext)}
   </title>
   <pubDate>${thing._date.strftime('%a, %d %b %Y %H:%M:%S %z')}</pubDate>
-  <dc:date>${thing._date.isoformat()}-0700</dc:date>
+  <dc:date>${thing._date.isoformat()}</dc:date>
   <description>
     ${description()}
   </description>