Certain CSS level 4 selectors are incorrectly compressed

From https://github.com/sass/sass/issues/2497:

> I have a CSS level 4 selector in SCSS:
> 
> ```scss
> .someclass:nth-child(1 of .due-today) { 
> /* ... */
> }
> ```
> 
> When compressing the CSS via Sass, the result is something like…
> 
> ```css
> .someclass:nth-child(1of.due - today) {
> /* ... */
> }
> ```
> 
> …which obviously isn't working.