Fix broken test suite

@sstephenson / @mislav - it appears that the `rbenv` test suite has been broken since 060f141;  the small change in this pull request ensures it's green once more... see [this comment](https://github.com/sstephenson/rbenv/commit/060f141b2174f9ab53b7c2c248ad8f426ec5c125#commitcomment-5108025) for more detail about the root cause for the test failure... thanks!

---

For reference:

```sh
$ bats -t test
1..141
...
not ok 141 carries original IFS within hooks
# (in test file /.../rbenv/test/which.bats, line 28)
# command failed with exit status 1
$_
```