def call!(env)
      if (request = Request.new(env)).for_asset_file?
        Response.new(env, request.asset_file).to_rack
      else
        @app.call(env)
      end
    end
def call!(env)
      if (request = Request.new(env)).for_asset_file?
        Response.new(env, request.asset_file).to_rack
      else
        @app.call(env)
      end
    end
The real Rack call interface.
 if an asset file is being requested, this is an endpoint - otherwise, call
 on up to the app as normal