mime/multipart: sort header keys to ensure reproducible output

mime/multipart: sort header keys to ensure reproducible output

Adds a transparent sort to the mime/multipart package, which is
only used in the CreatePart func. This will ensure the ordering
of the MIMEHeader.

The point of this change was to ensure the output would be consistent
and something that could be depended on.

Fixes #13522

Change-Id: I9584ef9dbe98ce97d536d897326914653f8d9ddf
Reviewed-on: https://go-review.googlesource.com/17497
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>