Do not fail if multiple certificate issues (#304)

Fix another location of this problem.

element/date/classes/element.php

@@ -141,7 +141,7 @@ class element extends \mod_customcert\element {
             $customcert = $DB->get_record('customcert', array('templateid' => $page->templateid), '*', MUST_EXIST);
             // Now we can get the issue for this user.
             $issue = $DB->get_record('customcert_issues', array('userid' => $user->id, 'customcertid' => $customcert->id),
-                '*', MUST_EXIST);
+                '*', IGNORE_MULTIPLE);
 
             if ($dateitem == CUSTOMCERT_DATE_ISSUE) {
                 $date = $issue->timecreated;