Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
It surprised me as well.  I just made the test pass from the actual output based on the current behavior.  `split` `to_s` and `<<` don't result in the same type of string.  maybe it should be looked at.  Also see @JonRowe in https://github.com/rspec/rspec-core/pull/1760#issuecomment-63763957 and my followup