Optimize percentile calculation by optimizing binary heap clone

lib/binary_heap.js

@@ -14,7 +14,7 @@ BinaryHeap.prototype = {
   clone: function() {
     var heap = new BinaryHeap(this.scoreFunction);
     // A little hacky, but effective.
-    heap.content = JSON.parse(JSON.stringify(this.content));
+    heap.content = this.content.slice();
     return heap;
   },