ZF-4453: Setting $_ignore as default value on <input type="submit"> and <button>
The ignore flag indicates that an element should be ignored when retrieving values at form level. (row: 694 in Element.php [Zend_Form_Element]).
Since one, as far as I know, almost never use the value of an input/button this would be a good default value. Makes it easier to run $row->setFromArray($form->getValues()); since MySQL complains about the non-existing column for the button/submit otherwise.