Problem: The Writer.CreatePart func in mime/multipart doesn't sort before writing headers.

Proposed solution is to obviously sort them and write them in the sorted order, allowing things like unit tests to accurately check if the resulting document is valid.

This is clearly a known issue as the source code has a comment stating this (https://golang.org/src/mime/multipart/writer.go#L97)

    97      // TODO(bradfitz): move this to textproto.MimeHeader.Write(w), have it sort
    98      // and clean, like http.Header.Write(w) does.
    99      for k, vv := range header {
   100          for _, v := range vv {
   101              fmt.Fprintf(&b, "%s: %s\r\n", k, v)
   102          }
   103      }

cc: @bradfitz