Fixed StreamWrapper::mkdir.

AWS-S3 is able to create a full path on a file_put_contents.
But If we run mkdir($path) before executing file_put_contents
the mkdir returns false.

With this patch, the mkdir always return true when a key is set.

![i have no iead](https://f.cloud.github.com/assets/408368/2160118/94c160f8-94b3-11e3-868a-f55cc89d9545.jpg)