Sometimes, esp. in tests, a double-slash occurs in the direct_fog_url.

This results in a 403 and means that the file can't be accessed.
From what I can tell, it results from the file's path being nil,
like when someone `store!`s a file manually in a test.
Merge pull request #73 from colinyoung/master

Fix: Sometimes, esp. in tests, a double-slash occurs in the direct_fog_url.