Allow save even if writeToURL is invoked in a background thread.
If the user chose to save anyway in response to a "The file has been changed by another application." prompt, the writeToURL… method
may be invoked in a background thread.
Reset back to f8963bcddff5da1aa02d831c5a16fb45c0406131
Merge branch 'projects/scuttlebutt/master' of https://github.com/adib/BSManagedDocument into projects/scuttlebutt/master