nebraska: Allow updatecheck request with event tag

I mistakenly thought we never send event tag with an updatecheck
request. But that is not correct, specially for the first update check
after a previous update/reboot. We should not fail on such condition.

This CL basically allows an request type UPDATE to have valid event
tag/attributes.

BUG=chromium:920404
TEST=unittest
TEST=send a request with both updatecheck and event to a running
nebraska.py and it accepted the request as valid.

Change-Id: Ie1a7d6f546e07c7df52e6520251130a55517283f
Reviewed-on: https://chromium-review.googlesource.com/1674246
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Nicolas Norvez <norvez@chromium.org>
2 files changed