Stringify keys in expiration commands

Keys are already stringified through the use of IndifferentHash. Unfortunately
expirations are stored in a separate data structure that treats strings and
symbols differently. Fixed by making all interactions with this structure
(expire_times) reference `key.to_s`, rather than `key`. This should correct the
behavior of not only EXPIRE and TTL, but all commands related to expiration.

Change-Id: Ie4be0cdc235b59cb1b764ffbc05fb4f0c0795802
Reviewed-on: https://gerrit.causes.com/19610
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Preston Guillory <preston.guillory@causes.com>