Merge: example: optional attribute with a default value

This one comes from a discussion with @R4PaSs

I'm not sure I want to really expose it as I still think that current constructors need more love and work.
So at least test the current behavior.

If you people have other alternatives, you can show them.

Pull-Request: #1614
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>