RSS feeds should use consistent date format, regardless of locale/timezone settings

During writing the IntelliJ Plugin I'm hitting the problem of Jira returning formatted dates in the RSS feed rather than a standard/consistent date format.

This means I'm going to have to add a setting to the plugin for the date format unless Jira spits out consistent date formats.

I'm not sure if the RSS spec specifies a particular date format, but if so, that should be used.