Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
Really, that was just to highlight the change in the PR in case you missed it, and also why I think the `pick_encoding` method should move over to the `EncodedString`.  Now that I know that this may still be an error on 1.9.2, I want to add a test for that.