title: How to pass response to a spider without fetching a web page?
body: <p><a href="http://doc.scrapy.org/en/latest/topics/architecture.html#downloader-middlewares" rel="nofollow">The scrapy documentation</a> specifically mentions that I should use downloader middleware if I want to pass a response to a spider without actually fetching the web page. However, I can't find any documentation or examples on how to achieve this functionality. </p>
  
  <p>I am interested in passing only the url to the request callback, populate an item's <code>file_urls</code> field with the url (and certain permutations thereof), and use the FilesPipeline to handle the actual download.</p>
  
  <p>How can a write a downloader middleware class that passes the url to the spider while avoiding downloading the web page?</p>