Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
I'm pretty sure that it's one of these two Encoding tests that failed on Ruby 1.9.2 https://travis-ci.org/rspec/rspec-support/jobs/45326387  I don't have that locally right now, will spin up a VM tomorrow