Add `header()` method for PHP services

[Marak]

PHP services should have the ability to manually set headers

see:

http://php.net/manual/en/function.header.php
https://github.com/bigcompany/hook.io/blob/master/bin/run-hook-php
https://github.com/bigcompany/hook.io/blob/master/bin/run-hook#L201