copy X-Delete-At unless X-Fresh-Metadata: true is supplied on an object copy

Current codes will copy metadata headers when x-fresh-metadata:false, we
still need copy "x-delete-at" header and ensure expiring work at the same
time.

Change-Id: Ie31326b5f7b565e51e5aa249279bc1786f7bc847
Fixes: bug #1067528
Merge "copy X-Delete-At unless X-Fresh-Metadata: true is supplied on an object copy"