Store a key, not the actual invite object, in the mail

The following situation causes the problem documented here: #2279851: drupal_test_email_collector variable can be set with an entity which, when unserialized, calls entity_get_info(), before it is defined.

invite_by_email_send_invitation(), defined in modules/invite_by_email/invite_by_email.module, adds the $invite object to the email $params:

$params = array('invite' => $invite)

The information is used later in the same file, in the function invite_by_email_mail():

$invite = $params['invite'];

To avoid this, we would do something like:

$params = array('invite' => $invite->iid);

$invite = invite_load($params['invite']);