This changes the behaviour of the XSRF tokens slightly:
  - timestamps are rounded up to the nearest ms (not ns)
  - the timestamp is appended to the taken after base64 encoding

Change-Id: Iaa32bd055bd76db77ef47f07b975e75c5ecaadf0
Reviewed-on: https://go-review.googlesource.com/17664Reviewed-by: David Symonds <dsymonds@golang.org>