fix: removed content-encoding header to avoid decoding errors

[adubovik]

httpx library used by openai is sending Accept-Encoding: gzip, deflate by default:

https://github.com/encode/httpx/blob/7b19cd5f4b749064c6452892a5e58a3758da1d1b/httpx/_client.py#L305-L315

This results in responses from the upstream which are encoded via gzip and having Content-Encoding: gzip header.

In the adapter, all the responses from upstream are decoded, modified and sent to the user as plain text, without any encoding.

Therefore, we must either do the encoding when needed or return the response without Content-Encoding: gzip header pretending that the server doesn't support encoding.

Given that the latter is so much easier to implement and that the adapter only proxies the response headers for errors, it was chosen as a solution.

How to reproduce the bug

Send an erroneous request to the dial-adapter deployment:

    response = client.chat.completions.create(
        model=MODEL,
        messages=[{"content": "2+3=?", "role": "users"}],
        stream=True,
    )
    for chunk in response:
        print(chunk)

Observe the error message:

  File ".venv/lib/python3.11/site-packages/httpx/_models.py", line 805, in read
    self._content = b"".join(self.iter_bytes())
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/httpx/_models.py", line 824, in iter_bytes
    decoded = decoder.decode(raw_bytes)
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/httpx/_decoders.py", line 78, in decode
    raise DecodingError(str(exc)) from exc
httpx.DecodingError: Error -3 while decompressing data: incorrect header check

resulted from an attempt to decode plain-text content using gzip algorithm since Content-Encoding header is set to gzip in the response.