Description:
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The development server appears to discard any HTTP requests that have the "Upgrade" header as malformed. I couldn't find information on what HTTP spec the server is built to support, if any. Actually implementing the HTTP upgrade protocol seems overkill for the scope of the server. Having it at least accept the request and ignore the upgrade offer I think is still spec compliant and preferable to generic discard and socket close. Alternatively, responding with HTTP/BAD_REQUEST and documenting the behavior is a more transparent option.

I have confirmed that my client was indicating HTTP/1.1 so the 'Upgrade' header should still be legal on the request. I don't actually need to use HTTP/2, I had a Java client that I was testing which requested an upgrade by default and this was a difficult issue to debug.

Tested on PHP/8.0.3 (Native Windows & alpine docker), PHP/8.0.8 alpine docker.

Test script:
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Succeeds: curl "http://localhost:8081" --trace-ascii - -X GET

Fails "Invalid request (Malformed HTTP request)": curl "http://localhost:8081" --trace-ascii - -X GET -H "Upgrade: HTTP/2.0" -H "Connection: upgrade"