CAS-1439: PGT and PT ids should begin with their assigned prefix

Supersedes https://github.com/Jasig/cas/pull/420

Issue https://github.com/Jasig/cas/issues/482 and https://github.com/Jasig/cas/issues/483
Reviewing this once more. you're right. I'll revert the change. My thought was to keep hashcode and equals close together for clarity, but given that STs and TGTs inherit from the same class, it's not possible to enforce strict type checking at the parent level.