RecordIO Stream should return `Async::Ready(None)` instead of `Async::NotReady`

[jeschkies]

According to the Tokio Chat example we should not return Async::NotReady unless the inner stream does. We violate this in client.rs:67. We should return Async::Ready(None) instead.

[jeschkies]

This contradicts a little https://docs.rs/futures/0.1.21/futures/stream/trait.Stream.html#associatedtype.Error. According to the docs a stream is considered finished when it returns Async::Ready(None).