attributes: falsey to unset data-attributes on divs? am I the only one having this problem maybe? What I do wrong?

The issue I had was that when `node.setAttribute('disabled',false)` is called on `<select>` tag it works great and removes the attribute ---- doing a similar thing on a `<div>` or any other tag results in `<div disabled="false">`

I fixed this for myself by making this change to mithril.

Before this change to mithril I would get this:
![screenshot 2014-07-17 12 56 07](https://cloud.githubusercontent.com/assets/1404558/3618683/75bb7556-0dec-11e4-86d1-acbc134bbc1b.png)
![screenshot 2014-07-17 12 56 19](https://cloud.githubusercontent.com/assets/1404558/3618687/7b3659ba-0dec-11e4-8f14-4f3b0bd5680f.png)

But what I wanted was this:
![screenshot 2014-07-17 12 55 19](https://cloud.githubusercontent.com/assets/1404558/3618693/83f4d356-0dec-11e4-8e68-1fcd7e910790.png)
![screenshot 2014-07-17 12 55 48](https://cloud.githubusercontent.com/assets/1404558/3618695/88305ce2-0dec-11e4-878f-c9ca47f12ae7.png)

Basically all so I could do things like `[data-disabled]` and other custom attributes in css for toggling things instead of confusing the css class namespace with boolean existence things ... but maybe this is something that should be done with css classes ..... or something else ....