ability to access http/1.1 preserved header casing

[GlenDC]

Is your feature request related to a problem? Please describe.

In #2574 it was clarified how the http/1.1 preserve casing feature works.
However I would like to have access to this private extension for purposes other than writing to a connection.

The order of headers already seems to be preserved when reading the headers of a request, which is awesome.
However it would be even better if I could also read the headers exactly as they were read, so including the casing.

Describe the solution you'd like

Be able to read header data as close to it being read from the wire as possible.
The header order already seems to be preserved, but the casing is not resected.
It might be possible already to preserve this for proxy reasons,
but it is not yet possible for .headers() reading out purposes.

Describe alternatives you've considered

The only other alternative I would see is log this data via an external tool like Wireshark,
but that brings in additional complexity, which I had hoped to solve with hyper directly.

[seanmonstar]

This is desired. The next step is to come up with a proposed public API. I'm tracking this in #2695, so I'll close here :)

[GlenDC]

Cool. Thanks a lot :)