Fix Array#flatten to discard Iterator::Stop

[straight-shoota]

When calling Array#flatten with an iterator as part of the flattened collections, the resulting array would have Iterator::Stop as part of the element type.

Iterator::Stop is just an indicator for the end of an iterator and not part of its elements. So it must be filtered out when typing the flattened array. FlattenHelper failed to do that.
This patch refactors the implementation to be more like Enumerable.element_type which has a similar purpose just without flattening.