css.global doesn't support hover?

I've created a global css like (see [jsfiddle](https://jsfiddle.net/unn265ko/20/))
```
Glamor.css.global(".hello", {
  ":hover": { "opacity": 0.7 },
  "opacity": 0.3,
}
```
and it transforms to
```
".hello{opacity:0.3;:hover:[object Object];}"
```
but not
```
.hello {
  opacity:0.3;
}
.hello:hover {
  opacity:0.7;
}
```
