bug: request body rewrite cannot automatically modify content-length request header.

[zxyao145]

Issue description

I am trying to rewrite the request body and response body , and the RequestFilter method code is roughly as follows:

origBody := []byte("{\"keyword\": \"xxxxx\"}")
log.Infof("request get Content-Length 1: %s", r.Header().Get("Content-Length"))
r.SetBody(origBody)
log.Infof("request get Content-Length 2: %s", r.Header().Get("Content-Length"))
r.Header().Set("Content-Length", strconv.Itoa(len(origBody)))
log.Infof("request get Content-Length 3: %s", r.Header().Get("Content-Length"))

and the log as follows:

request get Content-Length 1: 158
request get Content-Length 2: 158
request get Content-Length 3: 20

If I don't manually set the Content-Length, I will find that ResponseFilter method of the same plugin cannot be triggered and a timeout exception will occur.

Traceback (most recent call last):
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\site-packages\urllib3\connectionpool.py", line 467, in _make_request
    six.raise_from(e, None)
  File "<string>", line 3, in raise_from
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\site-packages\urllib3\connectionpool.py", line 462, in _make_request
    httplib_response = conn.getresponse()
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\http\client.py", line 1344, in getresponse
    response.begin()
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\http\client.py", line 307, in begin
    version, status, reason = self._read_status()
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\http\client.py", line 268, in _read_status
    line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
  File "C:\Users\xxx\AppData\Local\Programs\Python\Python38\lib\socket.py", line 669, in readinto
    return self._sock.recv_into(b)
socket.timeout: timed out

If this is by design, please update the documentation and the demo.

Environment

• APISIX Go Plugin Runner's version:
  github.com/apache/apisix-go-plugin-runner v0.5.1-0.20231128010119-a265bcd63ca1
• APISIX version:
  3.6.0
• Go version:
  1.19
• OS (cmd: uname -a):
  debian:bullseye-slim