__toString() should call __toString() as well if possible

There may be cases that an error occurs and another _toString()
implementation handles that appropriately. This is such a case.

library/Icinga/Web/Wizard.php

@@ -474,6 +474,6 @@ class Wizard
      */
     public function __toString()
     {
-        return $this->getForm()->render();
+        return (string) $this->getForm();
     }
 }