fix strftime('%z%Z') of gmtime

In my environment, `localtime->strftime('%z %Z')` returns `+0900 JST`, it's OK.

But `gmtime->strftime('%z %Z')` also returns `+0900 JST`, it seems fault.
The right result would be `+0000 UTC` (or `GMT`, `strftime(3)` on Linux represents `GMT` for `gmtime`).