Changed csrf-var to csrf-param

Changed csrf-var to csrf-param due to meta validation error. The validation error as mentioned in the issue https://github.com/yiisoft/yii2/issues/2363. It is fixed in this pull request.
Update BaseHtml.php
Update Request.php
Update yii.js
Update CHANGELOG.md
Update CHANGELOG.md
Merge pull request #2376 from dilip-vishwa/patch-2

Changed csrf-var to csrf-param