Prevent population of Submit elements

When populating a form with data using `$form->setData()`, the value of a submit element (an instance of `Zend\Form\Element\Submit`) will be populated with the value that was submitted with the form.

This PR prevents population of submit elements.

See [ML](http://zend-framework-community.634137.n4.nabble.com/Populating-a-submit-element-tp4660309.html).