rewrite_asset_path

# File actionpack/lib/sprockets/helpers/rails_helper.rb, line 146
        def rewrite_asset_path(source, dir, options = {})
          if source[0] == //
            source
          else
            source = digest_for(source) unless options[:digest] == false
            source = File.join(dir, source)
            source = "/#{source}" unless source =~ /^\//
            source
          end
        end