text: Title: "Transfer-Encoding" header is ignored
  Body: The "Transfer-Encoding" header is ignored by the requests library.
  
  ## Expected Result
  
  I expect a request in line with the following core data:
  
  >    POST /test.php HTTP/1.1
  >    Host: [replaced]
  >    Transfer-Encoding: chunked
  >    Content-Type: application/x-www-form-urlencoded
  >
  >    7
  >    param=2
  
  ## Actual Result
  
  I obtain a request without the "Transfer-Encoding" header despite it being explicitly set:
  >    POST /test.php HTTP/1.1
  >    Host: [replaced]
  >    User-Agent: python-requests/2.28.1
  >    Accept-Encoding: gzip, deflate, br
  >    Accept: /
  >    Connection: close
  >    Content-Length: 12
  >
  >    7
  >    param=2
  
  ## Reproduction Steps
  
  ```python
  import requests
  
  url = 'http://[replaced]/test.php'
  
  def data_chunks():
      yield b'7\r\n'
      yield b'param=2\r\n'
  
  response = requests.post(url,data=data_chunks(), headers={"Transfer-Encoding":"chunked", "Content-Type":"application/x-www-form-urlencoded"}, proxies={"http":"http://127.0.0.1:8080"})
  
  ```
  
  ## System Information
  
      $ python -m requests.help
  
  ```json
   {
    "chardet": {
      "version": "5.1.0"
    },
    "charset_normalizer": {
      "version": "3.0.1"
    },
    "cryptography": {
      "version": "38.0.4"
    },
    "idna": {
      "version": "3.3"
    },
    "implementation": {
      "name": "CPython",
      "version": "3.11.2"
    },
    "platform": {
      "release": "6.5.0-13parrot1-amd64",
      "system": "Linux"
    },
    "pyOpenSSL": {
      "openssl_version": "30000080",
      "version": "23.0.0"
    },
    "requests": {
      "version": "2.28.1"
    },
    "system_ssl": {
      "version": "300000d0"
    },
    "urllib3": {
      "version": "1.26.12"
    },
    "using_charset_normalizer": false,
    "using_pyopenssl": true
  }
  ```
  
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