ZF2-155: Message always includes empty CC header if no cc was specified

Description

Sending a message that never was assigned a CC address, still will include CC header and show it as empty.

Comments

It seems would happen because when SMTP::prepareRecipients does $message->cc() eventually the Message::getHeader function is called and it's setup if the queried header doesn't exist it creates it, and we would end up with the empty CC header despite no recipients ever being added to the header.