Allow for a Principal instance as the Context.SECURITY_PRINCIPAL in LoginInitialContextFactory

org.jboss.security.jndi.LoginInitialContextFactory does the following:

Object principal = env.get(Context.SECURITY_PRINCIPAL);

This code assumes the principal is just a String. It does not test to see if it's a Principal class implementation.
As a consequence, all principal objects which implement Principal won't work because it is not guarateed that their toString() method returns the principal name just like the getName() method does.