When Memcached Client (python) delete an item, the item's cas returned by delete operation is NOT "0" (since build 1758)

I use MemcachedClient API in mc_bin_client.py under testrunner/lib. Since build 1758, after deletion of an item, item cas value is NOT set to "0". It increase the cas value by 1

>>> import sys
>>> sys.path.append("lib")
>>> sys.path.append(".")
>>> import mc_bin_client
>>> client=mc_bin_client.MemcachedClient("10.5.2.23",11211)
>>> client.set("chisheng",0,0,"hong")
(4084912905, 9089085870612819, '')
>>> client.delete("chisheng")
(3079997155, 9089085870612820, '')