RFR 8071678: javax.script.ScriptContext setAttribute method should clarify behavior when GLOBAL_SCOPE is used and global scope object is null

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8071678/webrev.00/ for
https://bugs.openjdk.java.net/browse/JDK-8071678

Thanks,

-Sundar