RequestHandler sets its close callback on the HTTPConnection, not the…

… IOStream.

Fixes a bug in which close callbacks would never be called for subsequent
requests on a reused connection.

tornado/web.py

@@ -751,10 +751,10 @@ def finish(self, chunk=None):
 
         if hasattr(self.request, "connection"):
             # Now that the request is finished, clear the callback we
-            # set on the IOStream (which would otherwise prevent the
+            # set on the HTTPConnection (which would otherwise prevent the
             # garbage collection of the RequestHandler when there
             # are keepalive connections)
-            self.request.connection.stream.set_close_callback(None)
+            self.request.connection.set_close_callback(None)
 
         if not self.application._wsgi:
             self.flush(include_footers=True)