fixed NPE on retrieving user information from session

Hi once again,

execution of protected-josso/index.jsp resulted with NPE, because you've been retrieving user properties from request.getUserPrincipal().
User properties are now being retrieved from JOSSOSecurityContext

As as addition, I've put a piece of scriptlet just to demonstrate that user should be obtained only from JOSSOSecurityContext (getRemoteUser and getUserPrincipal return null)

Cheers