[-] Core : fixed bug with module mail_vars

The bug is that, for example in bankwire moudle we send the mailvars as template_vars from the validation controller within the module. But order history first receives the template vars, and then overwrite them with the default ones in the modules $this->extra_mail_vars.
This makes it impossible to create code that extends the mail_vars with conditional  mail_vars from an module validation controller.