Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
I think this is kind of fantastic, that we can, in one method, 

1. replace all invalid byte sequences with '?' and 
2. replace all undefined conversions with 
  1. their unicode equivalent, or, 
  2. if there are invalid byte sequences, with a string representation of their hex value