clicking "add" while selecting a folder opens rather than adds the folder

Might be a bug in the gnome file dialog, do not have the time to investigate
today, will do later.
On hoary, I tried adding a folder to an archive, but instead of adding the
folder, as the name would suggest, the button opened the folder

Dragging the folder into the window provided the solution, but the add button
should add folders too, since clicking on the folder would be the intuitive way
to open it anyway.