Reintroduced XmlSerializerCodec fix. Which was mistakenly removed.

Ramone/MediaTypes/Xml/XmlSerializerCodec.cs

@@ -2,20 +2,20 @@
 using System.Xml.Serialization;
 using System;
 using System.Collections.Generic;
-
+using System.Collections.Concurrent;
 
 namespace Ramone.MediaTypes.Xml
 {
   public class XmlSerializerCodec : XmlStreamCodecBase
   {
     // The XmlSerializer is thread safe according to the online docs, so it should be safe
     // to share instances.
-    static Dictionary<Type, XmlSerializer> Serializers { get; set; }
+    static ConcurrentDictionary<Type, XmlSerializer> Serializers { get; set; }
 
 
     static XmlSerializerCodec()
     {
-      Serializers = new Dictionary<Type, XmlSerializer>();
+      Serializers = new ConcurrentDictionary<Type, XmlSerializer>();
     }
 
 
@@ -38,11 +38,7 @@ protected override void WriteTo(object item, XmlWriter writer, WriterContext con
 
     protected XmlSerializer GetSerializer(Type t)
     {
-      if (!Serializers.ContainsKey(t))
-      {
-        Serializers[t] = CreateSerializer(t);
-      }
-      return Serializers[t];
+      return Serializers.GetOrAdd(t, CreateSerializer);
     }