[Mac] Fix DefaultCtorAllowed test for NSKeyedArchiver (#1748)

tests/introspection/ApiCtorInitTest.cs

@@ -145,6 +145,10 @@ namespace Introspection {
 		/// <param name="type">Type of the object, to be used if special logic is required.</param>
 		protected virtual void Dispose (NSObject obj, Type type)
 		{
+			//***** ApiCtorInitTest.DefaultCtorAllowed
+			//2017-01-23 15:52:09.762 introspection[4084:16658258] *** -[NSKeyedArchiver dealloc]: warning: NSKeyedArchiver deallocated without having had -finishEncoding called on it.
+			(obj as NSKeyedArchiver)?.FinishEncoding ();
+
 			obj.Dispose ();
 		}