Make sure we're not having troubles with headers

[chrisbetz]

I'm not sure multiple headers are handled correctly:

• case-insensitive
• possiblity to have the same header (case-insensitive!!!) multiple times.

in hato.client/response-map this might cause trouble, and I did not check other places.

:headers     (->> (.map (.headers response))
                     (map (fn [[k v]] (if (> (count v) 1) [k v] [k (first v)])))
                     (into {}))