Fix deadlock on saving an object with a circular reference.

[nlutsenko]

We were not removing the object itself when saving children, which simply task-deadlocked, as the save is already in progress for the root object.
This is following the same code as we have for saveEventually, where we explicitly remove root object from the list of unique objects to save, unless we were given an array.
Collection of dirty children will expand any PFObject into dictionary representation of it's estimatedData, so it's absolutely save to just pass a parse object as the root here.

Concrete example of the issue:

- obj2.save()
- obj2[key1] = obj1
- obj2.save()

---
- obj2[key2] = "randomValue"
- obj2.save() // It used to hang here...

Fixes #587.