No support for RSS 2.0 a10:updated

[daveaglick]

While this field isn't in the Harvard specification, I've seen it in the wild for RSS 2.0 feeds and the old SyndicationItem.LastUpdatedTime documentation references it: https://docs.microsoft.com/en-us/dotnet/api/system.servicemodel.syndication.syndicationitem.lastupdatedtime?view=netframework-4.7.1

It looks like RssParser.CreateItem() doesn't even support SyndicationItem.LastUpdated - it never gets set.

I propose support for <a10:updated> be added and that it set the existing SyndicationItem.LastUpdated property.