Chunked upload fails when using HTTP proxy

[sveint]

Not sure whether this is actually a requests bug, but trying here first as it's here it manifests.

I'm using a http proxy for https traffic, and it works fine for normal GET, POST and PUT requests using requests. However, if enabled chunked uploads, it fails with requests.exceptions.ConnectionError: [SSL: WRONG_VERSION_NUMBER] wrong version number (_ssl.c:1131)

From googling it appears to be trying to access the http proxy using https. I'm fully aware of the normal "problem" where people confuse the protocols and use "https://" for their http proxy, but as can be seen in the example below this is not the problem in this case.

I've been digging into adapters.py and the low_conn connection it uses appears to have the correct proxy set (with "http" as protocol), but this is outside my expertise.

Expected Result

Request to go through proxy like for non-chunked uploads without errors.

Actual Result

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/requests/adapters.py", line 521, in send
    low_conn.endheaders()
  File "/usr/lib/python3.8/http/client.py", line 1251, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/usr/lib/python3.8/http/client.py", line 1011, in _send_output
    self.send(msg)
  File "/usr/lib/python3.8/http/client.py", line 951, in send
    self.connect()
  File "/usr/local/lib/python3.8/dist-packages/urllib3/connection.py", line 414, in connect
    self.sock = ssl_wrap_socket(
  File "/usr/local/lib/python3.8/dist-packages/urllib3/util/ssl_.py", line 449, in ssl_wrap_socket
    ssl_sock = _ssl_wrap_socket_impl(
  File "/usr/local/lib/python3.8/dist-packages/urllib3/util/ssl_.py", line 493, in _ssl_wrap_socket_impl
    return ssl_context.wrap_socket(sock, server_hostname=server_hostname)
  File "/usr/lib/python3.8/ssl.py", line 500, in wrap_socket
    return self.sslsocket_class._create(
  File "/usr/lib/python3.8/ssl.py", line 1040, in _create
    self.do_handshake()
  File "/usr/lib/python3.8/ssl.py", line 1309, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL: WRONG_VERSION_NUMBER] wrong version number (_ssl.c:1131)

Reproduction Steps

Run script below with and without proxy. Without proxy it works fine, with http proxy it gives above error.

import requests

with open('test', 'w') as f:
    f.write('test')

with open('test', 'rb') as f:
    resp = requests.put('https://www.google.com', data=f)
    assert resp.status_code == 405

def x():
    yield "test".encode('utf-8')

resp = requests.put('https://www.google.com', data=x())
assert resp.status_code == 405

System Information

$ python -m requests.help

{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "3.0.1"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.4"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.8.10"
  },
  "platform": {
    "release": "4.18.0-425.10.1.el8_7.x86_64",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.28.2"
  },
  "system_ssl": {
    "version": "1010106f"
  },
  "urllib3": {
    "version": "1.26.14"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}

[sveint]

I did look first but missed the other issue. This is a duplicate of #6279