At a2119aca7dc82dc5b5cd40b1a2f56e82323da002 in go tip
because we want the bugfix at 821b54921a3cba5d853b531d4b03527c01bfc9b4
We could legitimately vendor as "vendor/mime/multipart" and shadow the
stdlib's but we do it in future for clarity.
Issue #642
Change-Id: Ifddbd4c9120936b8acc2f6ae31a97b1831b99f34