issue resolving url when url begins with #

The url resolver needs to be changed so it doesn't change urls that begin with a #

I've been running into an issue with trying to use bower-rails when using mapbox.js, 

they include a work around for IE8 clients in their css that has 
```css
behavior:url(#default#VML);
```

when the resolver runs it sees that and gets turned into 
```css
behavior:url(asset_path 'mapbox.js/#default#VML')
```

which then causes sprockets to fail because it can't find a file 'mapbox.js/' (#default#VML gets stripped out by sprockets), if I manually edit the generated erb file and change that line back to the original url(#default#VML) everything works fine and compiles.

The file in question is here:
https://github.com/gina-alaska/mapbox-dist/blob/master/mapbox.uncompressed.css#L767

Small rails app that demonstrates the problem
https://github.com/teknofire/test-bower-rails