Review request for JDK-8068872: Nashorn JSON.parse drops numeric keys

[Hannes Wallnoefer]

Please review JDK-8068872: Nashorn JSON.parse drops numeric keys:

http://cr.openjdk.java.net/~hannesw/8068872/

The main issue here was that ArrayData.ensure should only update the 
length if the new index is greater or equal to the old length.

I also fixed two other related issues:

- ScriptObject.defineProperty(int, Object) must call 
doesNotHaveEnsureDelete (which marks new unused slots as deleted) after 
having called ensure.
- There's no need to call ensure(length - 1) on a newly created 
ArrayData(length). I've removed two such cases.

Thanks,
Hannes