[merge][rfc] test that Transport.append sets permissions on existing files

[Martin Pool]

Martin Pool has voted reject.
Status is now: Vetoed
Comment:
After thinking about this more, I don't think we should add this test. 
It's accidental that this works on append at present, and it would be 
better to remove it.  Indeed as john said in his review, we should 
probably deprecate that parameter.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3Ce01316480708142355y111a032fu18637e7aa6a3a701%40mail.gmail.com%3E