Fixed saving for groups.

Store/GroupStore.cs

@@ -67,7 +67,7 @@ namespace HermesSocketServer.Store
         {
             List<Task> tasks = _chatters.Values.Select(c => c.Save()).ToList();
             tasks.Add(base.Save());
-            await Task.WhenAll(base.Save());
+            await Task.WhenAll(tasks);
         }
 
         protected override void OnInitialAdd(string key, Group value)