Fix zero time bug

Fix zero time bug.

`time.Time{}` needs to be able to round-trip everywhere. Previously
various things were implicitly adding the Local or UTC timezones, even
when the time value was zero.

R=vadimsh@chromium.org
BUG=https://github.com/luci/gae/issues/10

Committed: https://github.com/luci/gae/commit/97f1aaf90868a9a3bc651a808ceba3247145e202

[iannucci, 2015-09-19 18:26:51]

iannucci@chromium.org changed reviewers:
+ dnj@chromium.org, estaab@chromium.org, maruel@chromium.org,
tandrii@chromium.org

[iannucci, 2015-09-19 18:27:04]

On 2015/09/19 at 05:53:29, iannucci wrote:
> 

+ moar ppls

[dnj (Google), 2015-09-19 18:31:22]

dnj@google.com changed reviewers:
+ dnj@google.com

[dnj (Google), 2015-09-19 18:31:22]

lgtm

[iannucci, 2015-09-19 19:03:19]

The CQ bit was checked by iannucci@chromium.org

[commit-bot: I haz the power, 2015-09-19 19:03:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1345043005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1345043005/1

[commit-bot: I haz the power, 2015-09-19 19:04:55]

Committed patchset #1 (id:1) as
https://github.com/luci/gae/commit/97f1aaf90868a9a3bc651a808ceba3247145e202