[JRuby] Fix incorrect RUBY_ENGINE value 

fix the typo in `RUBY_ENGINE` value, so is matches RUBY_ENGINE.

```ruby
jruby-1.7.9 :001 > RUBY_ENGINE
"jruby"
```
 This way test is actually skipped on JRuby. let me know, if this needs to be changed to use `defined(JRUBY_VERSION)`. 

#11700