Bug in min-width and max-width matching with em values

We noticed a strange behavior when trying to do matches. Issue demostrated below:

```js
> const css = require('css-mediaquery')
undefined
> css.match('screen and (min-width: 48em)', { width: '48em' })
false
> css.match('screen and (max-width: 48em)', { width: '48em' })
false
```

I think these both should return true. That's how Chrome `matchMedia()`works when tested.