Fix SetSnapShot CopyTo variance failure

Author: fredericDelaporte

src/NHibernate.Test/UtilityTest/SetSnapShotFixture.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System.Collections;
+using System.Collections.Generic;
 using System.IO;
 using System.Runtime.Serialization.Formatters.Binary;
 using NHibernate.Collection.Generic.SetHelpers;
@@ -70,6 +71,17 @@ public void TestCopyTo()
 			Assert.That(list, Is.EquivalentTo(array));
 		}
 
+		[Test]
+		public void TestCopyToObjectArray()
+		{
+			var list = new List<string> { "test1", null, "test2" };
+			ICollection sn = new SetSnapShot<string>(list);
+
+			var array = new object[3];
+			sn.CopyTo(array, 0);
+			Assert.That(list, Is.EquivalentTo(array));
+		}
+
 		[Test]
 		public void TestSerialization()
 		{

src/NHibernate/Collection/Generic/SetHelpers/SetSnapShot.cs

@@ -114,12 +114,22 @@ IEnumerator IEnumerable.GetEnumerator()
 
 		void ICollection.CopyTo(Array array, int index)
 		{
-			if (!(array is T[] typedArray))
+			if (array is T[] typedArray)
 			{
-				throw new ArgumentException($"Array must be of type {typeof(T[])}.", nameof(array));
+				CopyTo(typedArray, index);
+				return;
 			}
 
-			CopyTo(typedArray, index);
+			if (array.GetType().GetElementType().IsAssignableFrom(typeof(T)))
+			{
+				if (_hasNull)
+					array.SetValue(default(T), index);
+				ICollection keysCollection = _values.Keys;
+				keysCollection.CopyTo(array, index + (_hasNull ? 1 : 0));
+				return;
+			}
+
+			throw new ArgumentException($"Array must be of type {typeof(T[])}.", nameof(array));
 		}
 
 		public int Count => _values.Count + (_hasNull ? 1 : 0);