mongodb: support custom ObjectID

diff --git a/lib/mongodb/bson/bson.js b/lib/mongodb/bson/bson.js
index 4194f5e..bf9094a 100644
--- a/lib/mongodb/bson/bson.js
+++ b/lib/mongodb/bson/bson.js
@@ -214,7 +214,7 @@ BSON.deserialize = function(data, is_array_item, returnData, returnArray) {
       // Read the oid (12 bytes)
       var oid = data.substr(index, 12);
       // Calculate date with miliseconds
-      var value = new exports.ObjectID(oid);
+      var value = ObjectID.createPk(oid);
       // Adjust the index
       index = index + 12;
       // Set the data on the object
@@ -500,9 +500,13 @@ ObjectID.prototype.generate = function() {
 };
 
 ObjectID.prototype.toHexString = function() {
+  return ObjectID.toHexString(this.id);
+};
+
+ObjectID.toHexString = function(id) {
   var hexString = '';
-  for(var index = 0; index < this.id.length; index++) {
-    var value = BinaryParser.toByte(this.id.substr(index, 1));
+  for(var index = 0; index < id.length; index++) {
+    var value = BinaryParser.toByte(id.substr(index, 1));
     var number = value <= 15 ? "0" + value.toString(16) : value.toString(16);
     hexString = hexString + number;
   }
@@ -520,7 +524,7 @@ ObjectID.prototype.__defineGetter__("generationTime", function() {
 
 ObjectID.index = 0;
 ObjectID.createPk = function() {
-  return new exports.ObjectID();
+  return new exports.ObjectID.apply(arguments);
 };
 
 ObjectID.createFromHexString= function(hexString) {