Various cleanups

Hi,

This class is used in [PHPMailer](https://github.com/PHPMailer/PHPMailer) and someone just pointed out your updated repo to me. While I was on that, I also did a bit of cleaning up. This PR contains:

* PHPDoc cleanups (`@var` is deprecated, fixed param types) - phpdoc now parses with no errors
* PSR-2 formatting cleanup
* Add missing `pre_content` property