Panicking following 0.3.17

It would seem that the patch brought forth some issues.

We panic at https://github.com/hyperium/h2/blob/af4bcacf6d3770e9e3dc10fdc631fc8c0bdd472b/src/proto/streams/streams.rs#L146, causing us to panic again at https://github.com/hyperium/h2/blob/master/src/proto/streams/streams.rs#L1359.