http2: avoid `Object.create(null)` for `Http2ServerRequest.headers`
Currently http2.Http2ServerRequest.headers returns headers which is initialized by Object.create(null) which is a different behaviour than what is seen in http.
Refs: https://github.com/nodejs/node/issues/29829
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines