Use a case insensitive URI Regexp for #asset_path

Context: https://gist.github.com/radar/5793814

The `URI_REGEXP` that various AssetUrl helpers use is currently case
sensitive when checking for a URI scheme. This means if you try to pass
a URL like `HTTP://www.example.com/path/to/image.jpg`, you end up with
a bogus asset path: `/assets/HTTP://www.example.com/path/to/image.jpg`.

URLs are case insensitive, so this regexp should be as well.

Signed-off-by: David Celis <me@davidcel.is>