I'd really like to see this not use stubbing if possible.  Given that ruby 2.0 has this behavior for enumerators, can't you just create an enumerator?  To me, this spec suggests that we're trying to support collections where the user has defined a `size` method to return `nil`...but I do'nt really have any interest on us supporting such a thing.  I'm only interested in support collections where `size` returns `nil`  insomuch as ruby 2.0 exhibits this behavior for enumerators -- so I'd like to the spec to simply create such an enumerator.  I'm fine having this spec only run on ruby 2.0 if need be.