In at2x, use the same background-size as non-retina background.

I had a problem with @2x plugin in rework, see:

```css
  background: var(black) url(../images/banner1.jpg) no-repeat center center;
  background-size: cover;
```

The @2x plugin changes the expected behaviour of the behaviour by setting background-size to `contain` for all @2x rules.

The provided commit uses whatever value was set by background-size property, otherwise `contain`.