Fieldset.php

<?php

class MyFieldset extends Fieldset implements InputFilterProviderInterface
{
  public function __construct()
  {
    parent::__construct();
    $this->add(array(
        'type' => 'Email',
        'name' => 'email'
    ));
  }
  
  public function getInputFilterSpecification()
  {
      return array(
        'email' => array(
            'required' => false    
        )    
      )
  }
}

FormTest.php

public function testCanOverrideDefaultInputSettings()
{
    $myFieldset = new MyFieldset();
    $form = new Form();
    $form->add($myFieldset);
    
    $inputFilter = $form->getInputFilter();
    $this->assertFalse($inputFilter->get('email')->isRequired());
}