Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
The constant hash is fine...I can see that it will have fewer string allocations.

I don't think I like the trailing `=`, though -- it looks odd.  I'd prefer to see no special character or perhaps wrap it in `<>` or similar (e.g. `<U+00A0>`).