Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
Also, I took a look at that file and it's quite large (over 2000 lines).  What specifically should I be looking for in that file to understand the `UNDEF_FALLBACK` logic?  (Because I don't understand it yet, but admittedly I haven't worked this deeply with encodings before).