"string"[true] is treated like "string"[1]

```
./bin/mruby -e "p 'bar'[nil]" 
"b"
```

```
./bin/mruby -e "p 'bar'[true]"
"a"
```

After fix:

```
./bin/mruby -e "p 'bar'[true]" 
nil
```