NH-3946: Linq where is base class doesn't get subclasses. Added failing test

Author: alexDevBR

src/NHibernate.Test/Linq/WhereTests.cs

@@ -652,6 +652,16 @@ where o is Dog
 			Assert.That(query.Count, Is.EqualTo(2));
 		}
 
+		[Test(Description = "NH-3946")]
+		public void PolymorphicSearchOnObjectTypeWithIsKeyword()
+		{
+			var query = (from o in session.Query<Animal>()
+						 where o is Mammal
+						 select o).ToList();
+
+			Assert.That(query.Count, Is.EqualTo(3));
+		}
+
 		[Test]
 		public void BitwiseQuery()
 		{