FilterIterator expects Iterator not Traversable

As outlined in https://github.com/guzzle/iterator/pull/2 and https://github.com/guzzle/iterator/commit/eb743f5622252ac3c3441bab203e3e3f9268f581#commitcomment-4009637 there is a problem with input types.

As this was reported against the wrong repository, here reported against the right repository first introducing the regression test showing the code currently fails.

This PR has been split, the parts about ChunkedIterator are move into #417