Define #symbolize_keys for Rails 3.2

Rails 3.2 calls symbolize_keys on hashes passed in to url_for. Since we pass (sanitized) TaintedHash instances into url_for in a variety of places, we need to make sure we behave as we should when this method is called.

This mirrors the behaviour of AS::HWIA#symbolize_keys.

cc @technoweenie @github/rails3