Merge hiccup symbol attrs into unmounting attrs

[PEZ]

To stop the unmounting transition from nuking the hiccup symbol id

• See #30

I couldn't figure out how to test the change to the get-unmounting-attrs function directly in any nice way, because I didn't see how to create a vdom from hiccup. Tested the change via the unmounting-test instead.

As I do not actually know what I am doing, the change could be totally bonkers of course. I do not understand the significance of ns when getting the headers for instance. Anyway, maybe I am going in the right direction, at least?