Fix fatal error "$string must be of type string, array given", happen…

…s when subject fields in certs contains multiple-values.

lib/Horde/Crypt/Smime.php

@@ -543,7 +543,7 @@ public function certToHTML($cert)
                  htmlspecialchars(
                      isset($fieldnames[$key]) ? $fieldnames[$key] : $key
                  ),
-                htmlspecialchars($value)
+                htmlspecialchars(implode(', ', (array)$value))
             );
         }
         $text .= "\n";