Normalize datetimes to account for tz

This patch makes sure that datetimes in the auth_token middleware
are normalized to account for timezone offsets.

Some of the old tests were changed to ensure that the expires string
stored in the cache is in ISO 8601 format and not a random float.

Fixes bug 1195924

Change-Id: I5917ab728193cd2aa8784c4860a96cdc17f3d43f
Merge "Normalize datetimes to account for tz"