Infinite loop when inserting Array into RBTree

Somewhat of an edge case, but can lead to a bad state in applications using this library -- hanging and high CPU usage. 

Repro case:
```
let rbtree = new RBTree(function(a, b) { return a.value - b.value; });
rbtree.insert( [ ] );
rbtree.insert( [ ] );  // <-- infinite loop
```

This works ok with `BinTree` though.