Bug 1032371 - Display users who have completed task on task detail page

As the `userid` is only used if username is `None`, you could eliminate this line and change line 103 to be:

`return self.get_queryset().get(user=self.kwargs.get('id'))`

Although you might want to check that userid is not `None` first, in which case you would just move this line inside the `if username is None:` statement, and then add the additional `if`.