simplenews_template_mail_alter() is wrongly changing every mail and removing the first letter of plain text notifications

I was noticing that messages (for instance, simplenews subscription confirmation message) was being sent properly but with the first letter removed. After debugging the code I noticed the problem was on this module. simplenews_template_mail_alter() was processing all emails (this hook is called for every email sent by drupal_mail()) and, by processing it by the _simplenews_template_mail_alter_simplenews_node() function, was doing a lot of unwanted thing to the message that definitely does not belong to a newsletter node.

I have modified the code so that only newsletter nodes are processed by this module and not the rest of the modules.

Attached is a patch for it.
a.=