making the config method of email-transport-classes mergable

At the comment of CakeEmail::config method, it is explained that it can merge new configuration into existing values.

But the configuration of the  email-transport-classes are initialized when CakeEmail::config method is used.
The following code does not work.

    $email = new CakeEmail('smtp');
    $email->config(array('log' => true));
    //SmtpTransport::_config is overwrote by using $default value.

This change make the config method of email-transport-classes mergable into the current config value.