Asset helpers should understand scheme-relative URLs

If you have this configuration:

ActionController::Base.asset_host = "assets.example.com"

And do this:

image_tag("//mysite.com/images/image.png")

You get something like:

<img src="http://assets.example.com//mysite.com/images/image.png" >

When instead this should be rendered:

<img src="//mysite.com/images/image.png" >

Which allows the browser to resolve the URL relative the current scheme, e.g. http, https.

An example of this case can be seen in the URI spec here:

http://labs.apache.org/webarch/uri/rfc/rfc3986.html#relative-normal

Ex:

"//g" => "http://g"

assuming the current scheme is http.

I've attached a patch to actionpack to support scheme-relative URL's in asset helpers.

Github commit can be seen here:

https://github.com/chadk/rails/commit/61eae0f2b1bfcb54f0355b710808b...