Fixing a bug when trying to parse keys from recursive GET.

When executing a recursive list of keys, the parent folder is being included as a result. However, as it is a folder, the value is set is to None. This is breaking when trying to base64 decode the value. The bug is fixed by only parsing the values that are known to be strings.
Simplifying a None-check and adding recursive get tests.
Merge pull request #6 from imgix/master

Fixing a bug when trying to parse keys from recursive GET. Thanks!