Allow to set body width less than 320px

I update karma from 0.13 to 1.7.0 and my unit tests have stopped work properly now. My unit tests contain next code:

```
beforeEach(() => {
document.body.style.width = '100px';
});
```
but for some reason in karma 1.x `body` width can not be less than 320px and because of this my unit tests are failing now.