01-index.tar not repopulated if no updates present

[snoyberg]

Perhaps I'm misusing the library, but I believe this is the current behavior of Hackage Security:

1. Perform a check, get all of the latest files
2. Delete 01-index.tar manually
3. Run a check again

Expected: it downloads 01-index.tar

Actual: I believe hackage-security is checking the other metadata .json files, determining that no updates are present, and exiting, ignoring the missing 01-index.tar file.