Added filter date_to_rfc822

According to the W3C RSS Feed Validator, feeds must format dates as described
on RFC-822 to be valid. Refer to their site to get more info:
http://feedvalidator.org/docs/error/InvalidRFC2822Date.html

I also added a couple of missing unit tests to the other date filters
Fixed expected output from xmlschema and rfc822
Merge pull request #892 from rafaelrosafu/master

Added date_to_rfc822 used on RSS feeds