CiviCRM alterMailParams hook does not work on subject

When trying to use civicrm hook_civicrm_alterMailParams to alter the email subject (e.g : replace custom tokens) , nothing will happen and your modifications to the mail subject will not have any affect on the sent email.

1. 1. The problem in action

if your myextension.php file contain this :

function myextension_civicrm_alterMailParams(&$params, $context) {
  $params['Subject'] = 'aaa';
  $params['html'] = 'aaa';
}

and no matter what the original email subject and body is , you should recienve an email with subject 'aaa' and body 'aaa' but what you will actually get is an email with only body = 'aaa' and the subject will equal the original assgined to it from UI.

This PR fixes this so in the case above you will have both subject and body = 'aaa'