CompressUnitValues fails on background-position (3.0.1)

```
This was tested using version 3.0.1

If you give the following css:
  background-position:0 0;

It incorrectly minifies it to:
  background-position:0;

This causes the browser to effectively treat it as:
  background-position:0 50%;
```

Original issue reported on code.google.com by `daniel.r...@gmail.com` on 15 Oct 2013 at 3:55
