Fix session_set_save_handler error #365 introduced with #340

Author: jfritschi

Diff for: .gitignore

@@ -8,3 +8,4 @@ utils/starting_HEAD
 # Composer
 composer.lock
 /vendor/
+/.buildpath

Diff for: source/CAS/Client.php

@@ -1113,7 +1113,10 @@ public function getChangeSessionID()
     public function setSessionHandler(\SessionHandlerInterface $sessionHandler)
     {
         $this->_sessionHandler = $sessionHandler;
-        return session_set_save_handler($this->_sessionHandler, true);
+        if (session_status() !== PHP_SESSION_ACTIVE) {
+            return session_set_save_handler($this->_sessionHandler, true);
+        }
+        return true;
     }
 
     /**