Handle both object instance and identifier passed to the "old" parameter of ManyToOneType.IsModified (#1499)

* Fixes #1496

Author: druidroad

src/NHibernate.Test/Async/NHSpecificTest/GH1496/AuditEventListener.cs

@@ -0,0 +1,38 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by AsyncGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+
+using System.Collections.Generic;
+using NHibernate.Event;
+
+namespace NHibernate.Test.NHSpecificTest.GH1496
+{
+	using System.Threading.Tasks;
+	using System.Threading;
+	public partial class AuditEventListener : IPostUpdateEventListener
+	{
+
+		public async Task OnPostUpdateAsync(PostUpdateEvent @event, CancellationToken cancellationToken)
+		{
+			if (isActive == false)
+			{ return; }
+
+			var modifiedItems = await (@event.Persister.FindModifiedAsync(@event.OldState, @event.State, @event.Entity, @event.Session, cancellationToken));
+			foreach (int index in modifiedItems)
+			{
+				ModifiedItems.Add(new Item
+				{
+					Index = index,
+					OldState = @event.OldState[index],
+					State = @event.State[index]
+				});
+			}
+		}
+	}
+}