"Transfer-Encoding" header is ignored

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
}
```

<!-- This command is only available on Requests v2.16.4 and greater. Otherwise,
please provide some basic information about your system (Python version,
operating system, &c). -->


> 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
> }
> ```
> 
> <!-- This command is only available on Requests v2.16.4 and greater. Otherwise,
> please provide some basic information about your system (Python version,
> operating system, &c). -->
> 

The version you are using is outdated, please upgrade to the latest version:

![IMG-20240708-WA0036.jpg](https://github.com/psf/requests/assets/129309925/4ded8782-5b57-44ad-a48f-1c33626a6614)

This will solve the issue.