Fixed: incorrect WSGI start_response signature.

This would cause error such as "<lambda>() takes exactly 2 arguments (3 given) error" when something like Sentry's raven tried to provide the optional exc_info argument.

geventwebsocket/handler.py

@@ -49,7 +49,9 @@ def run_websocket(self):
         try:
             self.server.clients[self.client_address] = Client(
                 self.client_address, self.websocket)
-            self.application(self.environ, lambda s, h: [])
+            def start_response(status, headerlist, exc_info=None):
+                return []
+            self.application(self.environ, start_response)
         finally:
             del self.server.clients[self.client_address]
             if not self.websocket.closed: