Revert removal of session_cache_limiter()

It was removed in [4a6159c].
The session_cache_limiter() is required for IE8 to correctly
hold onto sessions.

Fixes #2781

lib/Cake/Model/Datasource/CakeSession.php

@@ -602,6 +602,8 @@ protected static function _startSession() {
 				$_SESSION = array();
 			}
 		} else {
+			// For IE<8
+			session_cache_limiter("must-revalidate");
 			session_start();
 		}
 		return true;