Allow customizing header casing

[svix-jplatte]

Currently, the http crate contains a HeaderName type that is used in the implementation of HeaderMap as the key type. This HeaderName type stores all header names as lower case (for performance reasons), such that it is impossible¹ to (a) send headers with custom casing for the name and to (b) inspect the casing of headers for a request or response that was parsed into http::Request<_> / http::Response<_>.

In theory this shouldn't matter, because HTTP header names are specified to be case-insensitive. However in practice there are unfortunately many non-compliant implementations that don't work if you don't use the casing they expect.

Would you consider changes to the http crate that allow representing case-sensitive header names? I don't particularily care whether this would be done through some change to the HeaderName type, adding generics in some place, or even something entirely different, but I'd be happy to talk through different approaches if there is a general interest in solving this at the http crate level.

¹ without hacks such as abusing request / response extensions to store information about the casing, which hyper optionally supports on top of the http crate, c.f. hyperium/hyper#2695