JCLOUDS-619

Hello,

this is the fix for the issue described here[1].

Please review and let me know if you need something else.

Thanks and kind regards
Markus

[1] https://issues.apache.org/jira/browse/JCLOUDS-619
Can you rewrite this more simply as:

```
return String.format("%s%s%0" + base + "d", key, PART_SEPARATOR, partNumber);
```