Dewdrop/Fields/Helper/InputFilter's addInput doesn't always respect a field's required value

[matthewmcdonald]

I've discovered that for many-to-many relationships, the $input returned here always has required set to true, even when $field->isRequired() is false:

public function addInput(FieldInterface $field)
{
    $callback = $this->getFieldAssignment($field);
    $input    = call_user_func($callback);

I've fixed this locally by adding this change immediately afterwards:

if ($field instanceof DbField && 
    $input instanceof Zend\InputFilter\Input && 
    $input->isRequired() != $field->isRequired()
) {
    $input->setRequired($field->isRequired());
}

Is there a reason why $input->isRequired() would ever not be equal to $field->isRequired()?