Media download bug

[eob]

Hi folks -- thanks for this library!

This is a PR that fixes what I'm pretty sure is a non-working endpoint for media downloads.

Google appears to suggest that the URL scheme in master ought to work:

https://storage.googleapis.com/download/storage/v1/b/BUCKET_NAME/o/OBJECT_NAME?alt=media

But in my experience it just doesn't -- perhaps I'm doing something wrong.

Switching to this scheme for media downloads, however, seems to resolve the issue:

https://storage.googleapis.com/storage/v1/PATH_TO_RESOURCE

[nathanfallet]

Fixed by #45