In the related changes, we switched to using
Timestamp.normal representations for the X-If-Delete-At header.
However, the object-server required that the header be an int,
and the trailing '.00000' would cause trip the
"Bad X-If-Delete-At header value" error handling.
Now, we'll convert both the expirer header and the stored X-Delete-At to
Timestamps, even though we expect them to have no fractional value.
Note that we *could* have changed the expirer to continue sending
headers that are valid ints, but Timestamps are already the normal
Swift-way of passing and comparing times -- we should use that.
Related-Change: Ibf61eb1f767a48cb457dd494e1f7c12acfd205de
Related-Change: Ie82622625d13177e08a363686ec632f63d24f4e9
Change-Id: Ida22c1c8c5bf21bdc72c33e225e75fb750f8444b
57b632fbb5