Canvas: group.type should use app of first task. Closes #1516

celery/canvas.py

@@ -432,6 +432,11 @@ class group(Signature):
 
     def __repr__(self):
         return repr(self.tasks)
+
+    @property
+    def type(self):
+        return self._type or self.tasks[0].type.app.tasks[self['task']]
+
 Signature.register_type(group)