saml2_Message: Add unique SessionIndex for each SP.

Fixes issue 44.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2146 44740490-163a-0410-bde0-09ae8108e29a

modules/saml2/lib/Message.php

@@ -589,8 +589,7 @@ class sspmod_saml2_Message {
 		$sessionLifetime = $config->getInteger('session.duration', 8*60*60);
 		$a->setSessionNotOnOrAfter(time() + $sessionLifetime);
 
-		$sessionIndex = $session->getSessionIndex();
-		$a->setSessionIndex($sessionIndex);
+		$a->setSessionIndex(SimpleSAML_Utilities::generateID());
 
 		/* Add attributes. */