gcs: fix need for elevated permissions on SetModTime - fixes #3493
gcs: fix need for elevated permissions on SetModTime - fixes #3493

Before this change we used PATCH on the object to update the metadata.

Apparently this requires the "full_control" scope which Google were
unhappy with in their oauth review.

This changes it to update the metadata by copying the object ontop of
itself (which is the way s3 works).  This can be done with normal
permissions.

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone