validate endpoint should also return username

 src/main/java/org/keycloak/protocol/cas/endpoints/ValidateEndpoint.java

@@ -46,7 +46,10 @@
    }

    protected Response successResponse() {
        return Response.ok(RESPONSE_OK).type(MediaType.TEXT_PLAIN).build();
        StringBuilder sb = new StringBuilder(RESPONSE_OK);
        sb.append(clientSession.getUserSession().getUser().getUsername());
        sb.append("\n");
        return Response.ok(sb.toString()).type(MediaType.TEXT_PLAIN).build();
    }

    protected Response errorResponse(CASValidationException e) {