// it returns the data read before the error and the error itself (often io.EOF).
// ReadBytes returns err != nil if and only if the returned data does not end in
// delim.
// For simple uses, a Scanner may be more convenient.
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.