bugLogIf(ctx, errInvalidArgument)
		return errInvalidArgument
	}

	nv := make(textproto.MIMEHeader, len(v))
	for k, kv := range v {
		// Canonicalize all headers, to remove any duplicates.
		nv[http.CanonicalHeaderKey(k)] = kv
	}

	// Save all supported headers.
	for _, supportedHeader := range supportedHeaders {
		value, ok := nv[http.CanonicalHeaderKey(supportedHeader)]
		if ok {