ratelimit + done throws an error

Consider:

```
highland([1, 2, 3]).ratelimit(1, 1000).each(function () {}).done(function () {});
```

This code throws:

```
Uncaught Error: Can not write to stream after nil
```

inside of consume.

The issue _appears_ to be that we are emitting `nil` before we emit 3.

Reproduce the issue with the ratelimit(1, 1000), each, and done example, then inspect the consume path and the ratelimit completion handling. Done means the example completes without the “Can not write to stream after nil” error and does not emit nil before the value 3.

Written by the indexing model from the issue text.