Header name lowercase requirement is excessive

It was pointed out in https://code.djangoproject.com/ticket/32586 that the initial ASGI requirement that header names in responses MUST be lowercased - inherited from ASGI 1, I believe - doesn’t actually match the Django codebase, and upon inspection, this is because, unfortunately, a very small number of HTTP clients are case-sensitive to headers like Content-Type.

I believe the appropriate action is to relax the MUST requirement, and say that servers can/should try to preserve case when passed to them, but nothing requires it; this saves us from entirely flipping the expectation here. Thoughts?

[pgjones]

I’ve a request not to lowercase the incoming headers for a use case whereby Hypercorn is used as a proxy and the casing is sensitive downstream. Not lower-casing, does however cause some bugs e.g. https://github.com/pgjones/hypercorn/issues/77. I’m not sure what the least unpleasant solution is.

[andrewgodwin]

Yes, this is not for incoming request headers, but for outgoing request headers. I agree that all incoming ones should be lowercased; it’s the response headers that it seems we occasionally need to keep case sensitivity.

How about this wording instead:

Response header names are case-insensitive according to the HTTP RFCs, and we encourage ASGI applications to lowercase their response headers. However, some HTTP clients are case sensitive to headers like Content-Type, and so for this reason ASGI servers should make a best-effort attempt to preserve the case of response headers.