Fixed a possible bug regarding checking if a root directory exists

The following snippet seems to me a bit weird:

```php
<?php
vfsStreamWrapper::register();

var_dump(mkdir('vfs:///foobar')); // true
var_dump(is_dir('vfs:///foobar')); // false
var_dump(is_dir('vfs://foobar')); // true
```

This PR fixes this issue (as in making the first call to `is_dir` above return `true`), and does not break any existing tests.

There might be a possibility that I'm misunderstanding something, so if you feel the above code looks correct, disregard this PR. :)