This needs to clear out the `defaultproc` or something, otherwise you end up with different behavior for something like:

```
1.9.3p194 :001 > [1, 2, 3, 4, 5, 6, 7, 8].group_by(&:even?)
 => {false=>[1, 3, 5, 7], true=>[2, 4, 6, 8]}
1.9.3p194 :002 > [1, 2, 3, 4, 5, 6, 7, 8].group_by(&:even?)[12]
 => nil
```