fix; ability to use mongos for standalones (Fix #3537)

Author: vkarpov15

lib/drivers/node-mongodb-native/connection.js

@@ -45,7 +45,13 @@ NativeConnection.prototype.__proto__ = MongooseConnection.prototype;
 
 NativeConnection.prototype.doOpen = function(fn) {
   var server = new Server(this.host, this.port, this.options.server);
-  this.db = new Db(this.name, server, this.options.db);
+
+  if (this.options && this.options.mongos) {
+    var mongos = new Mongos([server], this.options.mongos);
+    this.db = new Db(this.name, mongos, this.options.db);
+  } else {
+    this.db = new Db(this.name, server, this.options.db);
+  }
 
   var self = this;
   this.db.open(function(err) {