Cannot correctly return response with 204 status and persist connection

[OlegTheCat]

(hunchentoot:define-easy-handler (return-empty-string :uri "/return-empty-string") ()
  (setf (htt:return-code*) 204)
  "")

(hunchentoot:define-easy-handler (return-nil :uri "/return-nil") ()
  (setf (htt:return-code*) 204)
  nil)

(hunchentoot:start (make-instance 'hunchentoot:easy-acceptor :port 4242))

The request to /return-empty-string returns the following:

> (drakma:http-request
 "http://localhost:4242/return-empty-string"
 :method :GET
 :keep-alive t :close nil)

NIL
204
((:CONTENT-LENGTH . "0") (:DATE . "Thu, 25 Oct 2018 14:46:51 GMT")
 (:SERVER . "Hunchentoot 1.2.38") (:KEEP-ALIVE . "timeout=20")
 (:CONNECTION . "Keep-Alive") (:CONTENT-TYPE . "text/html; charset=utf-8"))
#<PURI:URI http://localhost:4242/return-empty-string>
#<FLEXI-STREAMS:FLEXI-IO-STREAM {1001A7CE33}>
NIL
"No Content"

While it correctly returns NIL as a response body and persists connection, it returns Content-Length header, which violates the spec.

The request to /return-nil returns the following:

> (drakma:http-request
 "http://localhost:4242/return-nil"
 :method :GET
 :keep-alive t :close nil)
127.0.0.1 - [2018-10-25 14:49:59] "GET /return-nil HTTP/1.1" 204 - "-" "Drakma/2.0.4 (SBCL 1.4.6; Linux; 4.9.93-linuxkit-aufs; http://weitz.de/drakma/)"
""
204
((:DATE . "Thu, 25 Oct 2018 14:49:59 GMT") (:SERVER . "Hunchentoot 1.2.38")
 (:CONNECTION . "Close") (:CONTENT-TYPE . "text/html"))
#<PURI:URI http://localhost:4242/return-nil>
#<FLEXI-STREAMS:FLEXI-IO-STREAM {1001AF29F3}>
T
"No Content"

It returns an empty string as a response body (which is kinda OK), also it correctly doesn't return Content-Length header, but unfortunately closes the connection.

I'm not sure what's the correct way to achieve what I'm looking for, namely:

• the 204 response should not have Content-Length header
• the server should persist the connection
• the body should be empty (either NIL or empty string)

Can you please advice?