Fix checking for heredoc in mirb

## current
```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> print <<"EOD"
* "heredoc"
line 3: can't find string "EOD" anywhere before EOF
>
```

## fixed
```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> print <<"EOD"
* "heredoc"
* EOD
"heredoc"
 => nil
>
```