optimizer adding prefix to css URLs that start with '#'

This change makes it so that css URLs paths starting with '#' are not prefixed.  The only case where I've seen this crop up is the following which apparently enables VML in IE8.
```css
.lvml {
  behavior: url(#default#VML);
  /* ... */
}
```
One prominent library that uses this syntax is [Leaflet].  When building a project with CSS that imports Leaflet's CSS, e.g.:
```css
@import url("lib/leaflet.css");
```
the above statement becomes
```css
.lvml {
  behavior: url(lib/#default#VML);
  /* ... */
}
```

in the built CSS file.

[Leaflet]: https://github.com/Leaflet/Leaflet/blob/master/dist/leaflet.css