And some docs punctuation.

closes #1762

issue #1744

`yii\web\User::loginRequired()` now returns the `Response` object instead of exiting the application

Fixes #802: Added support for retrieving sub-array element or child object property through `ArrayHelper::getValue()`

After ensureBehaviors() call $this->_behaviors is an array already.
And since detachBehavior() unsets elements in $this->_behaviors, $this->_behaviors turns out to be an empty array after all iterations.