Retain asset query and fragment when generating hashed url

Resolves https://github.com/middleman/middleman/issues/1024

With `asset_hash` and `asset_host` enabled, `font-url('glyphs.eot?#iefix")` will now yield something like `/fonts/glyphs-a3d23.eot?#iefix`
That's what it took to get `font-url` working properly. I'm not sure why.