Fix pass callable object to sync_to_async

[Maillol]

Fix: Prevent passing callable objects that overwrite context

This fixes a regression where passing a callable object like huey.Task would break the context attribute.

After this change, huey.Task and similar callables can be used without overwriting context, restoring previous behavior.

[carltongibson]

Good example @Maillol. This looks right. I'll try and pull a point release together shortly.