Modify yii\data\Sort to work also with $_POST

Currently `Sort` works only with `$_GET` sort parameter.
Thus, for example combination of `GridView` and search form with method `post` does not work at all.

Proposed change is an attempt to make `Sort` accept `$_POST` also.

Example usage

Search model:

```
...
        $dataProvider = new ActiveDataProvider([
            'query' => $query,
            'sort' => [
                'defaultOrder' => [
                    'transaction_date' => SORT_DESC
                ],
                'dataMethod' => 'post',
                'formName' => $this->formName()
            ]
        ]);
...
```

GridView:
```
...
            'columns' => [
               ...
                [
                    'attribute' => 'transaction_date',
                    'sortLinkOptions' => [
                        'data-method' => 'post',
                        'href' => '#',
                        'onclick' => 'jQuery(\'#'.Html::getInputId($searchModel, $dataProvider->getSort()->sortParam).'\').val(jQuery(this).data(\'sort\')).closest(\'form\').submit();'
                    ],
                ],
...
```

Search form:

```
...
<?php echo Html::hiddenInput(Html::getInputName($model, $dataProvider->getSort()->sortParam), '', ['id' => Html::getInputId($model, $dataProvider->getSort()->sortParam)]); ?>
...
```