Kestrel: Add proper response for invalid header values.

[serdartepeyurt]

Describe the bug

There was an issue with the invalid header values on requests. It took a while for us to understand that because there was no indication of what went wrong. In our end, the problem was not using the UTF-8 encoding for headers on the client (Alamofire on iOS). I guess Kestrel only supports UTF-8 or vanilla ASCII, not extensions. That's okay but the problem was there were only Status 400 code and an empty response body.

To Reproduce

Fire up a kestrel server and send this header value with any request, with ASCII encoding => DEV_NAME, "Müge's iPad"

Exceptions (if any)

There were only Status 400 code and an empty response body.

Further technical details

.NET Core version is 3.1.404
It happens on both Windows and Linux Docker containers

[BrennanConroy]

Triage: We could consider adding a generic error message for all failures similar to IIS "something went wrong, look at the server logs to diagnose. <link to docs>"
We would need a way to opt-out of this for folks who hide the server they're using.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.