HTTP 响应也由四个部分组成,分别是:状态行、消息报头、空行和响应正文
HTTP/1.1 200 OK
Server: Tengine
Date: Sun, 03 Sep 2017 01:08:42 GMT
Content-Type: text/plain
Content-Length: 38
Last-Modified: Sun, 03 Sep 2017 01:08:21 GMT
Connection: keep-alive
ETag: "59ab5605-26"
Strict-Transport-Security: max-age=31536000;includeSubdomains;preload
X-Frame-Options: DENY
Accept-Ranges: bytes
CRLF 是回车换行的意思,CRLF CRLF 就是两个回车换行符