remove initial slash from existing keys. References #74

As mentioned on #74, I'm not 100% sure about this change, since I had to change two existing tests which explicitly required a slash at the beginning of the keys. I think there is a good change (about 80%) that this was a mistake. But it could also be due to some reason I ignore. So pick this pull request with caution.

In any case, I can confirm that after the change, things seem to be working on my end. Before it, Amazon plain refused to update files because of this.

Cheers!