Remove the haveItem check for RSS and Atom

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.
Merge pull request #20 from MJDSys/RemoveHaveItem

Remove the haveItem check for RSS and Atom