fix: 🐛 data attributes without value outputting as "true"

[kaisermann]

Closes #2803

[Conduitry]

I guess data attributes are always strings? I'm not sure whether it makes sense to distinguish between data-foo and data-foo={true}, nor whether that's even possible with the current AST. If not doing that is good enough for other attributes, it's probably good enough here.

[loilo]

Data attributes, just like all HTML attributes, are always strings. When having <div data-foo foo></div>, data-foo and foo both equal the empty string. IMO casting them to booleans is fine — it's an established practice and even done in the "official" implementations (e.g. with the checked, selected and disabled DOM properties).