Ignore Associations For OpenID2 (Google's Security Bug Fix)

I've had more requests elsewhere to submit a pull request to the base repo.

Fix for [Issue #51](https://github.com/openid/ruby-openid/issues/51). More information at https://groups.google.com/forum/?fromgroups=#!topic/google-federated-login-api/qXZDD7_K7jU.