Exporters read structure for format fields produces invalid 'required'

The read structure of an exporter does not produce the expected `required` field for optional formatted fields. The optional property of the formatted property is not applied to the format.

public static function define_properties() {

    return [

        'content' => [

            'type' => PARAM_RAW,

            'optional' => true,

        ],

        'contentformat' => [

            'type' => PARAM_INT,

            'optional' => true,

        ]

    ];

}

In the above example, the `contentformat` property will be marked as required. This is incorrect, it should be marked optional either from its own optional definition, or from its associated property.

Attached is a patch that demonstrates the issue, and offers a fix by referring to the parent property.