allow object keys for AVLTree

Author: rmanibus

lib/avltree.js

@@ -270,7 +270,7 @@ _AVLTree.prototype.insert = function (key, value) {
     // Same key: no change in the tree structure
     if (currentNode.compareKeys(currentNode.key, key) === 0) {
       if (currentNode.unique) {
-        var err = new Error("Can't insert key " + key + ", it violates the unique constraint");
+        var err = new Error("Can't insert key " + JSON.stringify(key) + ", it violates the unique constraint");
         err.key = key;
         err.errorType = 'uniqueViolated';
         throw err;