ClassReference: Add a default constructor

Trying to deserialize an instance of ClassReference would cause a crash,
due to a MissingMethodException.

This fixes the initial issue in the following bug reported against
Banshee: https://bugzilla.gnome.org/show_bug.cgi?id=672671

src/Mono.Upnp.Dcp/Mono.Upnp.Dcp.MediaServer1/Mono.Upnp.Dcp.MediaServer1/Mono.Upnp.Dcp.MediaServer1.ContentDirectory1/ClassReference.cs

@@ -33,6 +33,10 @@ namespace Mono.Upnp.Dcp.MediaServer1.ContentDirectory1
 {
     public class ClassReference : Class, IComparable<ClassReference>, IEquatable<ClassReference>
     {
+        protected ClassReference ()
+        {
+        }
+
         public ClassReference (bool includeDerived, string fullClassName)
             : this (includeDerived, fullClassName, null)
         {