Update hl_api_server.py

The proposed changes by @babsey

Author: vvorobjov

pynest/nest/server/hl_api_server.py

@@ -261,7 +261,7 @@ def do_call(call_name, args=[], kwargs={}):
         log(call_name, f"local call, args={args}, kwargs={kwargs}")
         master_response = call(*args, **kwargs)
 
-    response = [master_response]
+    response = [nest.serialize_data(master_response)]
     if mpi_comm is not None:
         log(call_name, "waiting for response gather")
         response = mpi_comm.gather(response[0], root=0)