ChunkedIterator 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.