Clean up activity recording (#722)

Author: blink1073

jupyter_server/services/kernels/kernelmanager.py

@@ -514,10 +514,11 @@ def record_activity(msg_list):
             self.last_kernel_activity = kernel.last_activity = utcnow()
 
             idents, fed_msg_list = session.feed_identities(msg_list)
-            msg = session.deserialize(fed_msg_list)
+            msg = session.deserialize(fed_msg_list, content=False)
 
             msg_type = msg["header"]["msg_type"]
             if msg_type == "status":
+                msg = session.deserialize(fed_msg_list)
                 kernel.execution_state = msg["content"]["execution_state"]
                 self.log.debug(
                     "activity on %s: %s (%s)",