Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
In any case, I think that comment is now out of date, since I've changed the implementation to make sane conversions.  See `\xa0` become `U+00A0=`  (example taken from the ruby test file)

```ruby
         # ISO-8859-1 -> UTF-8 -> EUC-JP
          # "\xa0" NO-BREAK SPACE, which is available in UTF-8 but not in EUC-JP
          context 'Encoding::UndefinedConversionError: U+00A0 to EUC-JP in conversion from ISO-8859-1 to UTF-8 to EUC-JP' do
            it 'replaces all bytes with a unicode representation in the format e.g. U+20EF=' do
              source_encoding = Encoding.find('ISO-8859-1')
              incompatible_encoding = Encoding.find('EUC-JP')
              string = "\xa0 hi I am not going to work".force_encoding(source_encoding)
              expect{ string.encode(incompatible_encoding) }.to raise_error(Encoding::UndefinedConversionError)
              resulting_string = build_encoded_string(string, incompatible_encoding)

              replacment_character = defined?(JRUBY_VERSION) ? '?' : 'U+00A0='                                                                                                     
              expect(resulting_string.to_s).to eq("#{replacment_character} hi I am not going to work")
```