Just a thought on this (probably should have mentioned this in the last review): what if users wanted a group called "UserDefaultGroup" or an owner named "UserDefaultOwner".  Is there a reason we don't use nil to signify "not specified" for these two properties?  Obviously the validation logic for each would need to change to `raise ArgumentError, "A non-empty [owner|group] must be specified." if value and value.empty?` and insync would just return true if nil.

There's probably some obvious reason for doing it this way that I'm just not seeing.