title: How to drop request in tornado?
body: <p>In order to test my client, I want to make some of the HTTP request sent by my client doesn't get any response.</p>
  
  <p>For example, I want every request should have cookie <code>topsession</code>, if not, no response.</p>
  
  <pre><code>class EchoHandler(tornado.web.RequestHandler):
      def get(self):
          if not self.get_cookie("topsession"):
              ###TODO
          else:
              self.write("0")
  </code></pre>