Kestrel: Add proper response for invalid header values.

### 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