Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
`trunk` is a branch, right?  So overtime this link may go out of date (such as if the file is renamed).  Can you make it a permalink using a tag or SHA?  If you want a SHA you can hit `y` and github will update your browser's URL with the currently viewed SHA...otherwise you can use a manually selected tag to pin to a particular ruby version.