call (Rack::ContentLength)

    # File lib/rack/content_length.rb, line 8
        def call(env)
          status, headers, body = @app.call(env)
          headers = Utils::HeaderHash.new(headers)

          if !Utils::STATUS_WITH_NO_ENTITY_BODY.include?(status) &&
             !headers['Content-Length'] &&
             !headers['Transfer-Encoding'] &&
             (body.respond_to?(:to_ary) || body.respond_to?(:to_str))

            body = [body] if body.respond_to?(:to_str) # rack 0.4 compat
            length = body.to_ary.inject(0) { |len, part| len + part.length }
            headers['Content-Length'] = length.to_s
          end

          [status, headers, body]
        end