Bug 15574. XML deserialization recursion: add array type to known_types?

One way to prevent https://bugzilla.xamarin.com/show_bug.cgi?id=15574 is to add the problematic array type to the `known_types`.

On the other hand, this might not be the intended use of `known_types`. In that case, perhaps a better fix would be to guard against recursion in the `foreach` loop:

	foreach (var t in kt.GetTypes (elementType))
		if (t != type)
			RegisterTypeAsKnown (t);