Use `encode()` to encode unwrapped lazy object

The unwrapped structure needs to go through the main encoding function again, because that handles plain Python types.

aleksis/core/util/core_helpers.py

@@ -273,6 +273,6 @@ def monkey_patch() -> None:  # noqa
     class DjangoJSONEncoder(json.DjangoJSONEncoder):
         def default(self, o: Any) -> Any:
             if isinstance(o, Promise) and hasattr(o, "copy"):
-                return super().default(o.copy())
+                return super().encode(o.copy())
             return super().default(o)
     json.DjangoJSONEncoder = DjangoJSONEncoder