The failing test is ModelFilterTest::testFilterArray(). It's not calling fixIdentifier directly and the following didn't work so I really don't know how to make the test work.
 
        $method = new \ReflectionMethod('\Sonata\DoctrineMongoDBAdminBundle\Filter\ModelFilter', 'fixIdentifier');
        $method->setAccessible(TRUE);

        $filter->filter($builder, 'alias', 'field', array(
            'type' => EqualType::TYPE_IS_EQUAL,
            'value' => array(new DocumentStub(), new DocumentStub())
        ));