RFR 8071989: NashornScriptEngine returns javax.script.ScriptContext instance with insonsistent get/remove methods behavior for undefined attributes

[A. Sundararajan]

Please review http://cr.openjdk.java.net/~sundar/8071989/ for 
https://bugs.openjdk.java.net/browse/JDK-8071989

PS. Piggybacking a sample script - getclassnpe.js - to detect 
obj.getClass() call for null checks. Ran that on nashorn code and fixed 
one more such instance which got missed in the last clean-up with 
JDK-8072595

Thanks,
-Sundar