Fix concurrency issue with "T" injection

--HG--
branch : dev

src/Orchard/Localization/IText.cs

@@ -1,5 +1,5 @@
 namespace Orchard.Localization {
-    public interface IText : ISingletonDependency {
+    public interface IText {
         LocalizedString Get(string textHint, params object[] args);
     }
 }

src/Orchard/Localization/LocalizationModule.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Reflection;
 using Autofac;
@@ -10,7 +11,7 @@ namespace Orchard.Localization {
         private readonly IDictionary<string, Localizer> _localizerCache;
         
         public LocalizationModule() {
-            _localizerCache = new Dictionary<string, Localizer>();
+            _localizerCache = new ConcurrentDictionary<string, Localizer>();
         }
 
         protected override void Load(ContainerBuilder builder) {