Fixes exception using is_dir on an object

StorageObject wasn't properly referencing the content type within Model.
This change also adds a test to ensure this not only gets tested but
that all possible combinations of dir-like content types and a non-dir
content type yield correct results.

Closes-bug: #20
Merge pull request #27 from briancline/is-dir-fix

Fixes exception when using is_dir on an object