Allow customizing header casing

首先检查 HeaderName 和 HeaderMap 类型，然后跟踪 header 名称在解析后的 http::Request 和 http::Response 值中是如何表示的。将可能的 API 更改与通过 hyper issue #2695 引用的现有基于 extensions 的变通方案进行比较；完成的标准是形成一份有文档记录的设计，支持对已发送 header 使用自定义大小写，并支持在解析后检查大小写。

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. https://github.com/hyperium/hyper/issues/2695