[Form] Choice types with choice_list option breaks on 2.7

Form choice type with `choice_list` option seems to be broken on rendering since Symfony 2.7.

See the concerned ticket: https://github.com/sonata-project/SonataAdminBundle/issues/2916

And the concerned type: https://github.com/sonata-project/SonataAdminBundle/blob/master/Form/Type/ModelType.php#L103-L115

Can you confirm?
