Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
Thanks. I'm switching back and forth between the two, so forgot my context :)

It actually behaves differently on 1.9.2.. https://travis-ci.org/rspec/rspec-support/jobs/41566833#L113

```plain
 1) RSpec::Support::EncodedString#to_s does nothing to an invalid byte sequence
     Failure/Error: expect(resulting_string.to_s).to eq("\xEF hi I am not going to work")
       
       expected: "\xEF hi I am not going to work"
            got: "\xEF hi I am not going to work"
       
```