Fix NRE in XmlSerializer to keep consistent behavior

Author: mythz

src/ServiceStack.Text/XmlSerializer.cs

@@ -108,6 +108,7 @@ public static void SerializeToWriter<T>(T value, TextWriter writer)
 
         public static void SerializeToStream(object obj, Stream stream)
         {
+            if (obj == null) return;
             using (var xw = XmlWriter.Create(stream, XWSettings))
             {
                 var serializer = new DataContractSerializer(obj.GetType());

tests/ServiceStack.Text.Tests/StringSerializerTests.cs

@@ -1,6 +1,7 @@
 #if !MONO && !IOS
 using System;
 using System.Collections.Generic;
+using System.IO;
 using Northwind.Common.ComplexModel;
 using Northwind.Common.DataModel;
 using NUnit.Framework;
@@ -50,6 +51,17 @@ public void Can_convert_to_Orders()
 			Serialize(dto);
 		}
 
+        [Test]
+        public void Can_serialize_null_object_to_Stream()
+        {
+            using (var ms = new MemoryStream())
+            {
+                JsonSerializer.SerializeToStream((object)null, ms);
+                TypeSerializer.SerializeToStream((object)null, ms);
+                XmlSerializer.SerializeToStream((object)null, ms);
+            }
+        }
+
 	}
 }