Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
Good question. This has to do with the way bytes combine in utf-8 to make characters.  I'm not sure what the ideal replacement string should be here, but I strongly agree with your implication that turning `hi` into `U+6968` isn't the right substitution.