if err == io.EOF {
			return errTrailerEOF
		}
		return err
	}
	switch rr := b.hdr.(type) {
	case *Request:
		mergeSetHeader(&rr.Trailer, Header(hdr))
	case *Response:
		mergeSetHeader(&rr.Trailer, Header(hdr))
	}
	return nil
}

func mergeSetHeader(dst *Header, src Header) {
	if *dst == nil {
		*dst = src
		return
	}
	for k, vv := range src {
		(*dst)[k] = vv
	}
}