The "data_class" option now must be set if a form maps to an object.

[pawaclawczyk]

• The "data_class" option now must be set if a form maps to an object. If you leave it empty, the form will expect an array, an instance of \ArrayAccess or a scalar value and fail with a corresponding exception.

  Likewise, if a form maps to an array or an instance of \ArrayAccess, the option must be left null now.

  Form mapped to an instance of Person:

  use Symfony\Component\OptionsResolver\OptionsResolverInterface;
  
  public function setDefaultOptions(OptionsResolverInterface $resolver)
  {
    $resolver->setDefaults(array(
        'data_class' => 'Acme\Demo\Person',
    ));
  }