[8u60] approval request for 8071989: NashornScriptEngine returns javax.script.ScriptContext instance with insonsistent get/remove methods behavior for undefined attributes

[A. Sundararajan]

Please approve.

Bug: https://bugs.openjdk.java.net/browse/JDK-8071989
jdk8u-dev webrev: http://cr.openjdk.java.net/~sundar/8071989/8u60/webrev.00/
jdk9 review thread: 
http://mail.openjdk.java.net/pipermail/nashorn-dev/2015-February/004161.html

Changes backported 'as is' except for modular source layout difference.

Thanks,
-Sundar