SplDoublyLinkedList: clone's behaviour should mimic PHP5

Discovered an issue in HHVM's reimplementation of SplDoublyLinkedList. It appears because it relies on SplDoublyLinkedNode's, a deep clone is required to prevent the clone from being mutated post creation.

In PHP, the SplDoublyLinkedList class comes from the SPL extension written in C. I suspect the divergence comes from reimplementing the class in PHP/HHVM, and cloning without special handling (since the internals are totally different)

I will follow up with a pull request 'fixing this' as best I see it. Raising the issue to remind me and to request feedback on this.