Fix chunk(n).eval bug

Patch in reference to https://groups.google.com/forum/#!topic/scalaz/k9-NP-BlHdA
An example of the bug can be seen by

```scala
Process.range(0, 6).chunk(4).map(Task(_)).eval.runLog.run.length ==
Process.range(0, 6).chunk(4).runLog.run.length
```