Extrude with both=True produces two solids i.s.o. one

[adam-urbanczyk]

The code below produces two solids i.s.o. one:

import cadquery as cq
from cadquery import exporters

points = [(20, 20), (20, 30), (30, 30), (30, 20)]

solid = (cq.Workplane("YZ")
         .polyline(points)
         .close()
         .extrude(distance=10., both=True))

It looks like combineWithBase does not fuse if there is no parent solid. I think we should change this behavior.