あ、抜けた
最初のCONNECTに対して串がレスポンス（200)してた(POSTが出る前）

あとこれに対する鯖からの応答
HTTP/1.1 206 Partial Content
Date: Fri, 09 Mar 2018 09:09:42 GMT
Content-Type: text/plain
Content-Length: 360
Connection: close
Set-Cookie: __cfduid=dbad819a6899eddfdd96bdbe3a968c8d51520586582; expires=Sat, 09-Mar-19 09:09:42 GMT; path=/; domain=.5ch.net; HttpOnly
Accept-Ranges: bytes
ETag: "5aa24eff-1a690"
Content-Range: bytes 107816-108175/108176
Last-Modified: Fri, 09 Mar 2018 09:08:15 GMT
Thread-Status: 1
User-Status: 1
Expect-CT: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct";
Server: cloudflare
CF-RAY: 3f8c677b4c7c6edb-NRT