http2: do no throw in writeHead if state.closed
The http1 implementation does not throw if the connection is down. The http2 compat implementation should do the same.
See: https://github.com/fastify/fastify-http-proxy/issues/51.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
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