Support for case-sensitive headers

[angel-langdon]

Checklist

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• I discussed this idea on the community chat and feedback is positive.

Is your feature related to a problem? Please describe.

I am currently serving PDFs using StreamingReponse and Range Requests (https://tools.ietf.org/html/rfc7233) specific headers to load them in batches using Mozilla pdf.js javascript library. The problem is that they have a case-sensitive implementation of headers, this is causing to fail because Starlette by default lower cases all headers.

I have tried disabling .lower() in Response class manually and everything works as expected:
https://github.com/encode/starlette/blob/master/starlette/responses.py

Describe the solution you would like.

A parameter in Response that could be inherited through all responses that did enable case-sensitive headers

Describe alternatives you considered

I have considered copying Reponse and StreamingResponse classes and customize them to don't lower case headers, however I don't think this is a long term solution.

Additional context

There are many libraries that consider headers to be case sensitive

[Kludex]

We are following the ASGI specifications.

There's already an open issue about this on the asgiref repository: django/asgiref#246

Once the specifications change there, we can comply here.