rgw: support S3 update of metadata
S3 metadata update is being done by copying of an object to itself with new metadata info.
rgw: obj copy respects -metadata-directive
Fixes #2542. The old behavior just merged src object attrs
and provided attributes. The new (and correct) behavior looks
at the x-[amz|rgw|...]-metadata-directive and either copies
the source attrs, or replaces them with the provided attrs.
Signed-off-by: Yehuda Sadeh <email@example.com>