fix issue with Ping iterating over a non thread-safe collection.

Author: craiggwilson

Driver/Core/MongoServer.cs

@@ -967,7 +967,7 @@ public virtual bool IsDatabaseNameValid(string databaseName, out string message)
         /// </summary>
         public virtual void Ping()
         {
-            foreach (var instance in _instances.ToArray())
+            foreach (var instance in Instances)
             {
                 instance.Ping();
             }