Incorrect minification for text-shadow

Hi,

I believe minification for `text-shadow` may be incorrect in some cases.

```bash
$ echo 'p{text-shadow:0 0 0;}' | uglifycss
p{text-shadow:0}
```

In that case, it should be `text-shadow:0 0`. According to [this page](https://www.w3schools.com/cssref/css3_pr_text-shadow.asp), _h-shadow_ and _v-shadow_ are required values.

Regards.