Removed fallback from mb_parse_str to parse_str from Request::getRestParams since mb_ availability is Yii2 requirement

Added Controller::enableCsrfValidation to support turning on/off CSRF validation for particular actions.

added version, csrfVar and csrfToken to yii js module.

To provider better REST API support PATCH method is added which is used
for partial updates, while a PUT should only update whole record.

issues #303 and yiisoft/yii#2664

Implemented Request::getAcceptedContentTypes().

This reverts commit 9d7597d6, reversing
changes made to 6b6390f7.