1c05b67e1c
it doesn't make sense to say structure='zip' if we are saving to an already instantiated FS object. The 'structure' keyword argument only makes sense when the destination is a path string, where we have a choice whether to create either a UFOZ (zipfile) or a normal UFO package. For an already instantiated FS object, we always write it out as a flat package, without the root contents subfolder.