copy rather than rename in Attachment#change_namespace

Change-Id: Iea77b0d93c01dfce92224ba6f500bcfa4c016f13
Reviewed-on: https://gerrit.instructure.com/31150
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>