Add tests for matchable attributes in RssFeed
Add tests for matchable attributes in RssFeed

Two of them are buggy. Tests check for buggy behaviour, with comments of
what's wrong and when it was broken.

der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat