The date the feed was last updated, as a string in the same format as it was published in the original feed.
This element is parsed as a date and stored in feed.updated_parsed.
Comes from
- /atom10:feed/atom10:updated
 - /atom03:feed/atom03:modified
 - /rss/channel/pubDate
 - /rss/channel/dc:date
 - /rdf:RDF/rdf:channel/dc:date
 - /rdf:RDF/rdf:channel/dcterms:modified