Possibility to specify headers encoding

[botsman]

Is your feature request related to a problem?

I have a case where I need to send some non-ASCII characters through headers.

import aiohttp
import asyncio

async def make_request():
    headers = {
        "some": "Serviços",
    }
    async with aiohttp.ClientSession() as session:
        async with session.get(
            "https://postman-echo.com/get",
            headers=headers,
        ) as response:
            assert response.status == 200
            json_body = await response.json()
            assert json_body["headers"]["some"] == headers["some"]  #  AssertionError

asyncio.run(make_request())

But the sent header value does not match with the actual value: Serviços

This is due to the fact that aiohttp uses utf-8 for encoding headers.

Describe the solution you'd like

It looks like it will be enough to pass "encoding" parameter to the http_writer._serialize_headers function through either aiohttp.ClientSession or aiohttp.ClientRequest (or any other better way).

Describe alternatives you've considered

Currently I had to inherit StreamWriter and ClientRequest in my code and overwrite StreamWriter.write_headers function for it to use latin-1.

Related component

Client

Additional context

If that feature looks fine then I could probably take it and send a PR.

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[Dreamsorcerer]

I have a case where I need to send some non-ASCII characters through headers.

This is not a valid HTTP message, and therefore we're unlikely to add anything that makes it easier for you to send invalid messages.

This is due to the fact that aiohttp uses utf-8 for encoding headers.

It should be percent encoded ASCII:
https://www.rfc-editor.org/rfc/rfc9110.html#section-5.5-4

If it's not, then that's a bug. The spec for that percent encoding says:

Producers MUST use the "UTF-8" ([RFC3629]) character encoding.
https://www.rfc-editor.org/rfc/rfc8187.html#section-3.2.1

Maybe it's possible to use the raw, pre-encoded header value, rather than using aiohttp to encode it, but it's not something we're likely to look at.