Updated AtomExtensions to create XmlSyndicationContent when ContentType is set to "xml"

Sharing the changes I had to make to support my use case: having content type="text/xml" in feed entry content.  Current code supports only plain text and html.  I updated AtomExtensions to create XmlSyndicationContent when ContentType is set to "xml".  Just sharing my changes - I'm not requesting a merge in right now as the changes were made only on the feed generation side and not for create/update operations.  Also, made a breaking change - IPublication.Content changed from string to object which could be handled more elegantly.