Fix ETag and Last-Modified content retrieval.
Fix ETag and Last-Modified content retrieval.

Before the feed URLs were always retrieved completely.

For my unrepresentative set of 123 feed URLs this gives another runtime
improvement of 25 % when doing a reload-all.

newsboat/newsboat,travisred/newsbeuter,x4121/newsbeuter,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,x4121/newsbeuter,newsboat/newsboat,der-lyse/newsboat,x4121/newsbeuter,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,x4121/newsbeuter,travisred/newsbeuter,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,travisred/newsbeuter,travisred/newsbeuter,der-lyse/newsboat,newsboat/newsboat