ConstructListObjInput function is incorrectly trimming the 'key' string

[BrianFanning]

I will open a PR shortly with a fix.

The key argument passed to ConstructListObjInput function is a path such as /my_folder/. The function trims the final / off the path, but instead it should remove the leading /. Prefixes do not start with a / character.

The trailing / should not be removed if you want to provide browse-style functionality. Take a look at the third example here from the official API docs

[almereyda]

As #54 is merged now, I suspect this here can be closed, too?