Golang MIME /多部分阅读器始终隐藏Content-Transfer-Encoding？

Golang mime/multipart library can be used to read and decode email messages.

I'm trying to make a proxy that is as transparent as possible, only changing those parts of message that it needs to.

Consider the input with a MIME part beginning

------=_Part_59911055_42259284.1503007218447
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

The below code fragment correctly sets the boundary and passes on the text/plain part.

However pEncoding is always read as the blank string "".

This would appear to be because of this

As a special case, if the "Content-Transfer-Encoding" header has a value of "quoted-printable", that header is instead hidden from this map and the body is transparently decoded during Read calls.

Is there a way to get the original value of the MIME part's Content-Transfer-Encoding?

// bodyCopy copies the mail message from msg to dst, with awareness of MIME parts.
// This is probably a naive implementation when it comes to complex multi-part messages and
// differing encodings.
func bodyCopy(dst io.Writer, msgHeader mail.Header, msgBody io.Reader) (int64, error) {
    var bytesWritten int64

    // Check Content-Type header - if not present, just copy mail through
    cType := msgHeader.Get("Content-Type")
    if cType == "" {
        return partCopy(dst, msgBody) // Passthrough
    }
    // Check what MIME media type we have.
    mediaType, params, err := mime.ParseMediaType(cType)
    cEncoding := msgHeader.Get("Content-Transfer-Encoding")
    log.Println(cEncoding)
    if err != nil {
        return bytesWritten, err
    }
    if strings.HasPrefix(mediaType, "text/plain") {
        return partCopy(dst, msgBody) // Passthrough
    }
    if strings.HasPrefix(mediaType, "text/html") {
        return htmlPartTransfer(dst, msgBody)
    }
    if strings.HasPrefix(mediaType, "multipart/") {
        mr := multipart.NewReader(msgBody, params["boundary"])
        for {
            p, err := mr.NextPart()
            if err != nil {
                if err == io.EOF {
                    err = nil // change EOF (which is normal exit from this loop)
                }
                return bytesWritten, err // EOF (normal) or error
            }
            pType := p.Header.Get("Content-Type")
            pEncoding := p.Header.Get("Content-Transfer-Encoding")
            log.Println(pEncoding)
            // TODO: need to handle various encodings of the parts
            pWrt := multipart.NewWriter(dst)
            pWrt.SetBoundary(params["boundary"])
            pWrt2, err := pWrt.CreatePart(p.Header)
            pWrt3 := quotedprintable.NewWriter(pWrt2) // Brute force method, converts everything back to QP. Don't want to do this!
            bytesWritten, err := partCopy(pWrt3, p)
            if err != nil {
                return bytesWritten, err
            }
            log.Printf("\t\tContent-type: %s = %d bytes
", pType, bytesWritten)
        }
    }
    return bytesWritten, err
}