Rename default to defaultValue for Opal

@adambeynon This should be unnecessary but I still have error when running with Opal master
> * The missing `$` from `default$` is now fixed on master. 

http://discuss.asciidoctor.org/Upgrading-Asciidoctor-js-to-the-latest-version-of-Asciidoctor-tp636p646.html