Make `display: inline-block` mixin work

Thanks for doing the initial work on this.

The reason the mixin didn't work is that the key in `index.js` didn't match the declaration name being modified. I had to merge the `display: flex` and `display: inline-block` login in order to make this work.

I also fixed tests along the way.

If you merge this into your repo, I can have a colleague of mine merge it upstream.