3.0 Some small Email fixes

A string would be of no use, the only place where `$_transport` is used, is in `reset()` and in `transport()`, and `transport()` can only set its value to an object instance, and as per documentation is supposed to return an instance of `AbstractTransport`.

Also this would cause the value to slip through [checks like `if (!$transport) {`](https://github.com/cakephp/cakephp/blob/3.0.0-beta2/src/Network/Email/Email.php#L1228), and it would "reset" the property to a value that is different from its initial state.