Thanks, so this fixes selectionbox overriding when neither selectionbox nor collisionbox was set in the passed table.
There's still a bug left. If you set the selectionbox and then the collisionbox, the custom selectionbox is overridden.

self.object:set_properties({
	selectionbox = {0,0,0, 1,1,1},
})
self.object:set_properties({
	collisionbox = {0.5,0.5,0.5, 1,1,0.7},
})

lt could be fixed by adding a boolean which becomes set to true if the selectionbox was set by the modder.