Update rss date format to RFC822 per the spec.

http://cyber.law.harvard.edu/rss/rss.html

"All date-times in RSS conform to the Date and Time Specification of RFC 822,
with the exception that the year may be expressed with two characters or four
characters (four preferred)."
Merge pull request #6 from Supermighty/master

Update rss date format to RFC822 per the spec.