add embedded extra field to versioning test

Author: Lewis Mohr

test/MsgPack.UnitTest/Serialization/VersioningTest.cs

@@ -64,9 +64,11 @@ private static void TestExtraFieldCore<T>( SerializationMethod method, EmitterFl
 				else
 				{
 					var packer = Packer.Create( stream, false );
-					packer.PackMapHeader( 4 );
+					packer.PackMapHeader( 5 );
 					packer.Pack( "Field1" );
-					packer.Pack( 1 );
+                    packer.Pack( 1 );
+                    packer.Pack( "Extra" );
+                    packer.PackNull();
 					packer.Pack( "Field2" );
 					packer.Pack( -1 );
 					packer.Pack( "Field3" );