Set indeterminate prop directly, fixes #128

[gmaclennan]

<input type="checkbox" indeterminate="indeterminate">

does not work currently, because indeterminate needs to be set directly instead of with el.setAttribute().

This should make things work as expected.

[goto-bus-stop]

Perrrfect, thank you! 📦 1.2.4

[gmaclennan]

@goto-bus-stop Having implemented this I see some drawbacks... this will not work for server-side rendering - there is simply no way to make it work. It also does not work with nanomorph or morphdom. I'm not sure what is needed to make it work. I found this react thread discussing this same issue where essentially they decided not to support it because of all the edge cases: facebook/react#1798

[goto-bus-stop]

Hmm, I see—we probably need to specialcase it in the updateInput function in nanomorph.

For server side render I suppose we will have to wait for the client side to render once to get the indeterminate property set, but having the attribute there probably doesn't hurt? If it starts to work once nanomorph runs I think it's OK to ignore SSR