Report object's sub-second last modified timestamp

[Timur Alperovich]

Swift currently returns two date headers for an object: X-Timestamp and Last-Modified. The X-Timestamp header is the time at which the object was created, down to the microseconds. It is not updated when POST is issued to update the metadata using "fast-POST". Last-Modified header only has second resolution, but does reflect the POST update of metadata on the object. Unfortunately, because the microsecond resolution is lost, the Last-Modified date may be in the future immediately after the object is created (as it rounds up to the nearest second).

It would be great if Swift exposed a header similar to X-Timestamp that reflected the actual POST time at the microsecond level.