optionally parse refresh tokens from authorization code exchange

workflow/request-token can be passed a response-parse-fn in place of
access-token-parsefn, e.g. for google’s oauth2

(defn response-parse-fn
  [req]
  (when-let [response (some-> req :body (chesire.core/parse-string
true))]
    {:access-token (:access_token response) :refresh-token
(:refresh_token response)}))