permission_required decorator should take any iterable if permissions

[Raphael Michel]

As it came up in the discussion on ticket #24914 on GitHub (​https://github.com/django/django/pull/4749#discussion_r31776720), there is no reason why permission_required only takes lists and tuples of permissions, while has_perm itself can take any iterable. To be consistent with the new mixins and other parts of Django where boths strings and iterables of strings are accepted (see e.g. model._meta.ordering), we should change this logic. I will prepare a pull request with the same logic that is used in other places.