Adapt a behavior of String#slice! to other Ruby implementations

Currently,  ```String#slice!``` returns ```nil``` value  when index (1st arg) equals self.size.
```
$ bin/mruby -e "p 'foo'.slice!(3,1)"
nil
```

But other Ruby implementations (such as CRuby, JRuby) return "" (null character).

```
$ ruby -e "p 'foo'.slice!(3,1)"
""
```