add download specs

This adds specs for every feature in download, except the progress proc. I had a lot of difficulty getting a quick progress proc call to work. I kept getting an error that `content_length` (on line 82 of download.rb) was nil so it couldn't use `/` in the if statement.

Not sure how to get that one working.

Very open to suggestions on these specs (and most things) since I still feel very unfamiliar with the spec world.