Avoid response buffering for ISeq response body

[kumarshantanu]

This PR makes StreamableResponseBody implementation for ISeq avoid response buffering, which is useful when the ISeq elements are delayed at source. This also allows for a smooth Server-sent Events flow.

Consider the example usage below:

(defn make-seq []
  (Thread/sleep #_10 1000)
  (str "data: " (System/currentTimeMillis) "\n\n"))

(defn handler [request]
  {:status 200
   :headers {"Content-Type" "text/event-stream"}
   :body (repeatedly make-seq)})

[weavejester]

Thanks for the update. Can you change the commit message to:

Flush response body stream when writing seqs

Update the StreamableResponseBody implementation for ISeq to flush the
OutputStream after each element. This prevents unnecessary delays when
dealing with blocking seqs, for example, when being used to implement
SSEs.

This makes it clearer which system is being affected by this change. There's no need to create a new PR for this - just --amend your commit message and then force push to update the PR.