public function test_filter_ref_array() {
    $obj = new stdClass();
    $a   = new MockAction();
    $tag = __FUNCTION__;

    add_action( $tag, array( $a, 'filter' ) );

    apply_filters_ref_array( $tag, array( &$obj ) );

    $args = $a->get_args();
    $this->assertSame( $args[0][0], $obj );
    // Just in case we don't trust assertSame().
    $obj->foo = true;
    $this->assertNotEmpty( $args[0][0]->foo );
  }