Fix default root size through font shorthand

Hi, 

CSS resets, as example Eric Meyer's one, uses the following rule:

```css
html {
  ...
  font: inherit;
  ...
}
```

The current version of pixrem nicely try to get predefined root font-size but throw the following error:

```js
Cannot read property '1' of null
```

I made a quick fix and also added a new spec to ensure coverage for cases like this.

Cheers!