fix expirer bug with unicode container listings

If the container names in the expirer's account are returned as
unicode strings (as is the case with some json libraries), the
expirer compared eg u'1' == '1', which is problematic. This patch
ensures that the unicode is coerced to ascii so the comparison
is correct.

Change-Id: I72b322e7513f7da32e8dc75c6bf0e7e016948c88
Merge "fix expirer bug with unicode container listings"