Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
Yes, e.g. `\xa0` becomes `U+00A0=`. I added the trailing `=` just because I thought it helped readability, but I'm not sold on it.

Also, having a constant like this is really just to avoid method dispatch and/or string allocation.  I'd be okay with changing this to a method.