func (b *Reader) ReadBytes(delim byte) ([]byte, error) {
	full, frag, n, err := b.collectFragments(delim)
	// Allocate new buffer to hold the full pieces and the fragment.
	buf := make([]byte, n)
	n = 0
	// Copy full pieces and fragment in.
	for i := range full {
		n += copy(buf[n:], full[i])
	}
	copy(buf[n:], frag)
	return buf, err
}