$stdin.tty? reports wrong value on JRuby-9.0.0.0-pre1

When stdin is a pipe, `$stdin.tty?` should return false. On JRuby-9.0.0.0-pre1 it returns true:

```
[matijs@walnut] rbenv shell jruby-9.0.0.0-pre1
[matijs@walnut] echo '' | ruby -e 'p $stdin.tty?'
true
[matijs@walnut] rbenv shell 2.2.0
[matijs@walnut] echo '' | ruby -e 'p $stdin.tty?'
false
[matijs@walnut] rbenv shell jruby-1.7.18
[matijs@walnut] echo '' | ruby -e 'p $stdin.tty?'
false
```
