Avoid Array to String Conversion in Aleph driver (#1540)

module/VuFind/src/VuFind/ILS/Driver/Aleph.php

@@ -1076,7 +1076,7 @@ class Aleph extends AbstractBase implements \Zend\Log\LoggerAwareInterface,
                     ],
                     null, 'POST', null
                 );
-                $due = (string)$xml->xpath('//new-due-date');
+                $due = (string)current($xml->xpath('//new-due-date'));
                 $result[$id] = [
                     'success' => true, 'new_date' => $this->parseDate($due)
                 ];