Fix imagebox:set_image not returning anything

set_image should always return true or false according to the docs

lib/wibox/widget/imagebox.lua

@@ -104,7 +104,7 @@ function imagebox:set_image(image)
     if self._private.image == image then
         -- The image could have been modified, so better redraw
         self:emit_signal("widget::redraw_needed")
-        return
+        return true
     end
 
     self._private.image = image