empower headers users in defining their own headers

[GlenDC]

This crate comes with a lot of utilities which aren't available in public API space. This leads to people having to define their own utilities for very similar headers. In case this crate would be ok in adding a lot more (semi) std http headers that would be less of an issue, but AFAIK it is intended to keep this crate limited to the more popular ones and certainly not X- like headers, even though plenty of them are pretty standard.

With utilities I mean CSV parsing, HttpDate, etc...
I'm fine with this being behind an unstable feature or w/e, but right now you pretty much need to copy them out which is a lot harder to keep in sync then just having your code break with an update.

Similar, a lot headers that are supported here have also severe limitations in how they can be used, making it now always easy or even possible to use the typed headers due to these limitations. E.g. ContentType only has support for a couple of content-types to construct them with, just to give an example.

All in all this crate (headers) is fantastic, but I feel it is currently a lot more limited in how it can be used as well as externally extended.