Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
> I thought adding the describe '#to_s' do was sufficient. Otherwise it requires a bit more investigation to understand

Well, there are other examples in this group where calling `#to_s` does not result in a string of all question marks, so clearly `it 'replaces all characters with either a ? or a unicode ?'` isn't the behavior you always see when `#to_s` is called.  Are `UTF-16LE` and `IBM737` completely incompatible encodings?  Is that what causes that?