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

// unreadDataSizeLocked returns the number of bytes of unread input.
// It returns -1 if unknown.
// b.mu must be held.
func (b *body) unreadDataSizeLocked() int64 {
	if lr, ok := b.src.(*io.LimitedReader); ok {
		return lr.N
	}
	return -1
}

func (b *body) Close() error {