[Swift] Fix 503 error when get queue stats
[Swift] Fix 503 error when get queue stats

when use swift backend, if there is only one message in a queue,
then try to get the queue 's stats, Zaqar will raise 503 error.

It's because that Swift backend used shallow copy which is wrong.

Closes-bug: #1686021

Change-Id: Ie563ff63ea90e7011a6ece525f5e7795109ba1d0

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar