Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
small picture: To make Rubocop happy.  Locally I need the encoding comment to get the tests to run, but rubocop says it's only required if there's utf8 chars in the code (utf8 chars in a comment is also a problem for rubocop)

big picture:  I'm just not sure if we want to provide a  hook for the replacement string.