Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
The first link is helpful.  I don't understand the reason you've put the 2nd link, though, given that it's for code that's no longer in rspec-expectations and not directly related to the method below (besides being related to encodings in general), as far as I can tell.  Can you explain, @bf4?