Avoid duplicating chains in chords (#3779)

* Avoid duplicating chains in chords

* Add Ryan Hiebert to contributors

CONTRIBUTORS.txt

@@ -231,3 +231,4 @@ Alejandro Pernin, 2016/12/23
 Yuval Shalev, 2016/12/27
 Morgan Doocy, 2017/01/02
 Arcadiy Ivanov, 2017/01/08
+Ryan Hiebert, 2017/01/20

celery/canvas.py

@@ -1282,6 +1282,9 @@ class chord(Signature):
             group_id=group_id, chord=body, root_id=root_id).results
         bodyres = body.freeze(task_id, root_id=root_id)
 
+        # Chains should not be passed to the header tasks. See #3771
+        options.pop('chain', None)
+
         parent = app.backend.apply_chord(
             header, partial_args, group_id, body,
             interval=interval, countdown=countdown,