Use straight edge for slots SlottedEdge

Used the connecting edge due to an aliasing issue. This broke as the
FingerJointEdge now places fingers even for smaller lengths.

Thanks to Nic Jansma (https://github.com/nicjansma) for reporting!

Resolves: #452

boxes/edges.py

@@ -817,7 +817,7 @@ class SlottedEdge(BaseEdge):
             if self.slots:
                 Slot(self.boxes, self.slots)(self.settings.thickness)
             else:
-                self.edge(self.settings.thickness)
+                self.boxes.edge(self.settings.thickness)
 
         self.edge(self.sections[-1])