Hashes do not need this monkey patch on ruby 1.9.3, 2.0.0, 2.1.3.

Subclasses of String with instance variables can be used as keys and do not lose
their instance variables.

The spec passes without the patch.
Merge pull request #884 from jrafanie/remove_patch_for_string_subclasses_as_hash_keys

Hashes do not need this monkey patch on ruby 1.9.3, 2.0.0, 2.1.3.