Use rss:pubDate over atom:updated when determining the posting date

this fixes an issue where feeds that mix rss and atom tags (i.e. youtube channel upload feeds) get an incorrect posting data if the updated time changes.

It might also be useful to place the dc:date tags higher in the order.

An example feed with this issue is http://gdata.youtube.com/feeds/base/users/OfficialNerdCubed/uploads?alt=rss