fix __globals__ example

Author: nggit

examples/__globals__.py

@@ -26,7 +26,8 @@ async def _on_request(self, **server):
     async def _on_response(self, **server):
         response = server['response']
 
-        del response.headers[b'x-debug']
+        if not response.headers_sent():
+            del response.headers[b'x-debug']
 
 
 app.logger.info('entering %s', __file__)