RFR JDK-8134364: Add defensive copies to get/set methods for OCSPNonceExtension

[Jamil Nimeh]

Hi all,

This is a quick fix to the OCSPNonceExtension class to add a couple 
defensive copies to public get/set methods.

JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8134364
Webrev: http://cr.openjdk.java.net/~jnimeh/reviews/8134364/webrev.00

Thanks,
--Jamil