Just putting this out there before I make a pull request.
Is there any interest in an asset_url method as a view helper?
It would work by using the config.action_controller.asset_host if it exists
and prepending this to the results of asset_path.
I have found this necessary when writing html emails and needing a full url
path for assets. Since my CDN in the asset_host mirrors my assets in my
app it makes perfect sense. Might not be a common enough use case to put
into core though.