RFR: 8034820: Wrong isAssignableFrom test when adding Principal to Subject

[Sean Mullan]

Please review this fairly trivial fix to correct an incorrect usage of 
Class.isAssignableFrom to check if a class is a superclass.

bug: https://bugs.openjdk.java.net/browse/JDK-8034820
webrev: http://cr.openjdk.java.net/~mullan/webrevs/8034820/webrev.01/
Thanks,
Sean