Heading attributes disappear from hierachical sections about djot.js

[jgm]

I've made a change which restores intended behavior, which is that the id attribute migrates the the section but other attributes stay on the heading.

I don't know if that's the best solution, though. Another approach would be to put all the attributes on the section.

[hellux]

Hmm, I would prefer to move all of the attributes to the section. I don't see why the id would be considered special in this case.

It would be kind of similar to how lists and list items are handled, attributes get applied to the list instead of the list item. If we want to let one apply attributes to the heading or list item, it could perhaps be solved similarly in both cases.

[jgm]

OK, I've done it that way.