rewrite_asset_path

# File actionpack/lib/sprockets/helpers/rails_helper.rb, line 142
        def rewrite_asset_path(source, dir)
          if source[0] == //
            source
          else
            source = digest_for(source) if Rails.application.config.assets.digest
            source = File.join(dir, source)
            source = "/#{source}" unless source =~ /^\//
            source
          end
        end