Due to recent changes involving how values are dealt with in xmlx, update
the RSS/Atom parsing. Instead of using the Value property of an xmlx Node,
use the new GetValue function offered in PR jteeuwen/go-pkg-xmlx#15.
The haveItem check for RSS and Atom causes the feeds to act unexpectedly.
For RSS, the checked tags don't necessarily have to be unique. For atom,
it is allowed to have duplicate items (including duplicated ids) in one feed,
so this shouldn't be stopped either.
In RSS feeds, the author of an item was always overwritten by a non-standard
creator tag. Change this so that creator is only used when it actually
appears. Otherwise use the previous value of Author, whatever that is.