[FIX] mass_mailing: on mass mailing sending, set same_thread for message composer

same_thread boolean field of mail.compose.message model define wether or not replies should be posted in the thread or not
On composer creation, this boolean should be passed according to the reply_to_mode of the mass_mailing
Otherwise, all replies would be posted in the thread, and the reply_to will be ignored (if same_thread id True, reply_to is poped from the mails values, mail/wizard/mail_compose_message.py:284)