fix: support callback with createConnection() and useMongoClient

Re: #6217

Author: vkarpov15

lib/index.js

@@ -188,14 +188,14 @@ var checkReplicaSetInUri = function(uri) {
  * @api public
  */
 
-Mongoose.prototype.createConnection = function(uri, options) {
+Mongoose.prototype.createConnection = function(uri, options, callback) {
   var conn = new Connection(this);
   this.connections.push(conn);
 
   var rsOption = options && (options.replset || options.replSet);
 
   if (options && options.useMongoClient) {
-    return conn.openUri(uri, options);
+    return conn.openUri(uri, options, callback);
   }
 
   if (arguments.length) {