Handle X-MJ-Vars as json string

[Nightbr]

See issue #5

In order to remove SwiftMailer error/warning when you try to pass an array of variables to X-MJ-Vars, I propose to json_encode when we set the header and in our BaseMessagePayload we json_decode in order to create the payload for the API properly.

Only tested against API v3.1 on my side.

Code Example:

    /**
     * Configure specific headers for message
     * @method configureHeaders
     * @param  \Swift_Message    $message
     * @return \Swift_Message
     */
    protected function configureHeaders(\Swift_Message $message, $templateId, array $vars)
    {
        //Configure Headers
        $headers = $message->getHeaders();

        $headers->addTextHeader('X-MJ-TemplateID', $templateId);
        $headers->addTextHeader('X-MJ-TemplateLanguage', true);
        $headers->addTextHeader('X-MJ-Vars', json_encode($vars));

        return $message;
    }

[latanasov]

Looking good to me.