Remove body from redirect responses

This change removes the anchor link from the body for redirect responses. There's no content-type set for these responses, and although this body is convenient it doesn't seem appropriate for a general purpose library. Our clients, for instance, don't expect to receive this kind of content from our API.

I've also changed the header key from `Location` to `location` as I think this is more appropriate for ring-compatible libraries.

Do these seem like reasonable changes?

I resisted doing this as part of this PR, but I also think that it would be good to remove the default `:headers {}` and `:body ""`. There's no reason to set these, and empty values don't fit very well with way Clojure writers typically check for existence using checks for truthiness (e.g. `(if (:body response) ...)`). What do you think?