Encode nil datastore keys as PTNull.

Encode nil datastore keys as PTNull.

This fixes a panic in serialize when it is asked to serialize a nil
*ds.Key, causing it to dereference its fields. Instead, a *ds.Key is
serialized as a PTNull entry like datastore.

Add some tests to assert proper conversion and serialization.

BUG=

Committed: https://github.com/luci/gae/commit/273b8ea01f0d03bd19cd4c14a9b1ba790d125306

[dnj, 2016-01-12 20:18:12]

dnj@chromium.org changed reviewers:
+ iannucci@chromium.org

[dnj, 2016-01-12 20:18:12]

PTAL

[iannucci, 2016-01-12 20:42:38]

lgtm

[dnj, 2016-01-12 21:10:50]

The CQ bit was checked by dnj@chromium.org

[commit-bot: I haz the power, 2016-01-12 21:10:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1576153003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1576153003/1

[commit-bot: I haz the power, 2016-01-12 21:15:18]

Description was changed from

==========
Encode nil datastore keys as PTNull.

This fixes a panic in serialize when it is asked to serialize a nil
*ds.Key, causing it to dereference its fields. Instead, a *ds.Key is
serialized as a PTNull entry like datastore.

Add some tests to assert proper conversion and serialization.

BUG=
==========

to

==========
Encode nil datastore keys as PTNull.

This fixes a panic in serialize when it is asked to serialize a nil
*ds.Key, causing it to dereference its fields. Instead, a *ds.Key is
serialized as a PTNull entry like datastore.

Add some tests to assert proper conversion and serialization.

BUG=

Committed:
https://github.com/luci/gae/commit/273b8ea01f0d03bd19cd4c14a9b1ba790d125306
==========

[commit-bot: I haz the power, 2016-01-12 21:15:21]

Committed patchset #1 (id:1) as
https://github.com/luci/gae/commit/273b8ea01f0d03bd19cd4c14a9b1ba790d125306