Added HTTP Status 422 (Unprocessable Entity) to CakeResponse

Quick fix to add HTTP Status Code 422 (inspired by Inspired by https://github.com/cakephp/cakephp/pull/1247). I wanted to use this code in my personal project but it resulted in a CakeException because it was not list in $_statusCodes.

    Error: [CakeException] Unknown status code
    Request URL: <hidden>
    Stack Trace:
       #0 <hidden>\vendor\pear-pear.cakephp.org\CakePHP\Cake\Network\CakeResponse.php(390): CakeResponse->statusCode(422)