gdk3: try to add constants in Gdk::Selection (not working)

The following error occurred:

    % ruby -r gdk3 -e 'p Gdk::Atom.intern("PRIMARY")'
    /.../gobject-introspection-2.2.1/lib/gobject-introspection/loader.rb: line 108
       GLib-GObject-CRITICAL **:g_boxed_copy: assertion 'G_TYPE_IS_BOXED (boxed_type)' failed
    nil