title: Scrapy spider logger doesn't recognize extra argument
body: <p>I tried to add some context information to Scrapy spider log message and send it to Logstash, like this:</p>
  
  <pre><code>def parse(self, response):
      self.logger.info('Status is %s' % response.status, extra={'status': response.status})
  </code></pre>
  
  <p>But it didn't work because I have a value from <code>extra</code> into <code>args</code> variable of LogstashFormaters(<code>args({'status': response.status})</code>.
  But if I use directly module like:</p>
  
  <pre><code>logging.info('Status is %s' % response.status, extra={'status': response.status})
  </code></pre>
  
  <p>It works well. What do I do wrong?</p>