Fix polymorphic serializer fails in specific situations in Unity.

Author: yfakariya

CHANGES.txt

@@ -657,3 +657,4 @@ Release 0.9.0 RC1 T.B.D
   * Fix asynchronous multi dimensional array deserialization corruption.
   * Fix enum serialization throws NullReferenceException in Unity. Issue #215.
   * Fix MessagePackSerializer.Capability does not work correctly in Unity.
+  * Fix polymorphic serializer error in Unity.

src/MsgPack/Serialization/Polymorphic/PolymorphicSerializerProvider`1.cs

@@ -1,4 +1,4 @@
-#region -- License Terms --
+#region -- License Terms --
 // 
 // MessagePack for CLI
 // 
@@ -46,6 +46,7 @@ public PolymorphicSerializerProvider( MessagePackSerializer<T> defaultSerializer
 		public PolymorphicSerializerProvider( SerializationContext context, MessagePackSerializer defaultSerializer )
 		{
 			this._defaultSerializer = MessagePackSerializer.Wrap<T>( context, defaultSerializer );
+			this._defaultSchema = PolymorphismSchema.Create( typeof( T ), null );
 		}
 #endif