Image extension is replicated when a URL is converted to a string more than once

If a URL has been given an extension, and then converted to a string, the extension will be added again the next time the URL is converted to a string:

```php
$url = $client->getImageUrl('id');
echo $url->png();
echo $url;
echo $url;
```

will give the following output:

```
http://imbo/users/christer/images/id.png?accessToken=<token>
http://imbo/users/christer/images/id.png.png?accessToken=<token>
http://imbo/users/christer/images/id.png.png.png?accessToken=<token>
```