Add specs for Array#values_at with weird Range

* https://github.com/rubinius/rubinius/pull/2490#pullrequestreviewcomment-5347303
* https://github.com/rubinius/rubinius/pull/2490#pullrequestreviewcomment-5347339
Fix Array#values_at with weird Range for 2.0
Merge pull request #2490 from kachick/2.0-array-values_at-with_weird_range

Array#values_at should always fill with nil when given weird Range from 2.0