Force 409 for post requests (untestet)

conflict-on-post.clj

(defresource conflict-on-post 
  :post! (fn [ctx] (if (detect-conflict ctx) { ::conflict true}))
  :respone-with-entity (fn [{conflict ::conflict}] (not conflict))
  :handle-no-content (fn [{conflict ::conflict}] 
                       (when conflict
                         (-> (as-response "my-regular-response")
                             (assoc :status 409) ;; force status
                             (ring-response)))))