ImmutableList, ImmutableSet should override Spliterator<E> spliterator()

[henohenotsuyoshi]

I think ImmutableList, ImmutableSet should override Spliterator<E> spliterator(). Currently default method in Collection is an implementation for both.

[TheLoneKing]

Why do we need to override? I think the default implementation in the ImmutableCollection class works for both ImmutableList and ImmutableSet.