title: Using meta attribute inside a downloader middleware
body: <p>According to the Scrapy docs, one cannot use <code>response.request</code> in a downloader middleware because request object will be attached to response only after passing all other downloader middlewares. Though I have noticed that in case of redirect (to captcha page), responses inside a downloader middleware have not only empty <code>request</code> field - but also empty meta (pycharm debugger tells me that the response is not related with any request). How could I force Scrapy to keep the meta while processing inside a downloader middleware? I have placed <code>meta=response.meta</code> to every request but still getting errors about missing meta keys - and absent meta attribute aswell.</p>
  
  <pre><code>    def start_requests(self):
  
          for value in values::
              yield Request(
                  self.SEARCH_URL,
                                  ),
                  meta={'ssomekey': value},
              )
  </code></pre>
  
  <p>From downloaders middleware:</p>
  
  <pre><code>    def process_response(self, request, response, spider):
  
          if not hasattr(response, 'meta'):
              print "there is no meta"
  </code></pre>
  
  <p>After launching immediately prints "there is no meta"</p>