[celery#305]: extend_task_props_callback relocated.

`extend_task_props_callback` moved from `_get_extended_properties`
to `_store_result`. Suggested by @AllesVeldman.

`extend_task_props_callback` will get the `request` object as first
parameter and a copy of `task_props` (avoiding potential manipulation of
the original `task_props`) as second paramenter.

---
Resolves celery#305

Fixes celery#314

django_celery_results/backends/database.py

@@ -79,10 +79,6 @@ def _get_extended_properties(self, request, traceback):
                 # task protocol 1
                 task_kwargs = getattr(request, 'kwargs', None)
 
-            # TODO: We assuming that task protocol 1 could be always in use. :/
-            extended_props.update(
-                extend_task_props_callback(getattr(request, 'kwargs', None)))
-
             # Encode input arguments
             if task_args is not None:
                 _, _, task_args = self.encode_content(task_args)
@@ -133,6 +129,9 @@ def _store_result(
             self._get_extended_properties(request, traceback)
         )
 
+        task_props.update(
+            extend_task_props_callback(request, dict(task_props)))
+
         self.TaskModel._default_manager.store_result(**task_props)
         return result