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
do we capitalize this in the protocol doc?  Let's match (i.e. should it be Proxy Granting Ticket)