SECURITY-787 Speed up SimpleRole#equals

We did some profiling of our JBoss AS instance and `SimpleRole#equals`
showed up as a hot method. A lot of time seems to be spent in
`java.lang.Class.cast(Object)`. Switching to a normal Java cast should
fix this since we're inside an `instanceof` and it always succeeds.

 * use normal cast in `SimpleRole#equals` instead of
   `java.lang.Class.cast`
 * add short circuit case for `this`

Issue: SECURITY-787
https://issues.jboss.org/browse/SECURITY-787
Merge pull request #18 from marschall/SECURITY-787

SECURITY-787 Speed up SimpleRole#equals