Avoid compressing time units.

This patch fixes an issue where Stylus compresses zero-duration time units.

E.g:

```sass
body
  animation: spin 1s linear 0s infinite
```

Becomes:

```css
body {animation: spin 1s linear 0 infinite}
```

According to the [CSS time unit specification](https://developer.mozilla.org/en-US/docs/Web/CSS/time) speficying time without a unit is invalid.