Currently when using fast-post, the manifest is updated with the given
'x-object-manifest' header on a POST. If no such header is supplied,
then the manifest will change to a regular object.
This is not currently true when using post-as-copy.
This patch changes the DLO POST using post-as-copy behavior to match
that of using fast-post. It was also documented that
'x-object-manifest' must be provided on a POST to a manifest file.
Change-Id: Ie1143ab1a2c8f8c21e258a36badbff5d947769d4
Closes-bug: 1612991