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
improved fixture command
gridview header/footer fix
Merge pull request #2384 from zelenin/gridview-header-fix

gridview header/footer fix
Merge pull request #2376 from dilip-vishwa/patch-2

Changed csrf-var to csrf-param
ar doc.
Merge pull request #2381 from Ragazzo/fixture_controller_imporved

improved fixture command
Fixes #2386
added note about usage to AR::instantiate()

fixes #2370
improved config
Merge pull request #2389 from Ragazzo/basic_app_mail_config

improved config
Renamed csrfParam back to csrfVar for consistency.
yii\mongodb\Collection::mapReduce() fixed to handle 'inline' output correctly.
view improved
fixed message