fix: set group should set user property (#51)

Author: Mercy811

src/amplitude/client.py

@@ -128,7 +128,8 @@ def set_group(self, group_type: str, group_name: Union[str, List[str]], event_op
             event_options (amplitude.event.EventOptions): Provide additional information for event
                 like user_id.
         """
-        event = IdentifyEvent(groups={group_type: group_name})
+        identify = Identify().set(group_type, group_name)
+        event = IdentifyEvent(groups={group_type: group_name}, user_properties=identify.user_properties)
         event.load_event_options(event_options)
         self.track(event)
 

src/test/test_client.py

@@ -194,6 +194,7 @@ def callback_func(event, code, message=None):
             self.assertEqual("test_user_id", event["user_id"])
             self.assertEqual("test_device_id", event["device_id"])
             self.assertEqual({"type": ["test_group", "test_group_2"]}, event.groups)
+            self.assertEqual({"$set": {"type": ["test_group", "test_group_2"]}}, event.user_properties)
 
         self.client.configuration.callback = callback_func
         for use_batch in (True, False):