ticket 2855 CakeEmail & Transports have ambiguous config behaviors

See http://cakephp.lighthouseapp.com/projects/42648/tickets/2855-cakeemail-transports-have-ambiguous-config-behaviors

This change would pick up the notion that the `CakeEmail` config is overwritten in  `_applyConfig()` with

    $this->_config += $config;

and extend it to the transport class with

    $this->transportClass()->config() + $config

The only problem is that `SmtpTransport::config()` has a slightly different behavior than `AbstractTransport::config()`.