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.
undefine symbolize_keys! since this is an indifferent access hash
Merge pull request #9 from github/rails-3-2-symbolize-keys

Define #symbolize_keys for Rails 3.2