The date this entry 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 entries[i].updated_parsed.
Comes from
- /atom10:feed/atom10:entry/atom10:updated
 - /atom03:feed/atom03:entry/atom03:modified
 - /rss/channel/item/pubDate
 - /rss/channel/item/dc:date
 - /rss/channel/item/dcterms:modified
 - /rdf:RDF/rdf:item/dc:date
 - /rdf:RDF/rdf:item/dcterms:modified