net/http: fix zombie connection leaked

[lesismal]

When Server.IdleTimeout is not zero and after the first request, the client sends part of the data of a full request, and the data is no less than 4 bytes, the idle timeout which has been set will be reset to zero here and would never close the conn:
https://github.com/golang/go/blob/master/src/net/http/server.go#L1996

Fixes #51614

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Patch Set 1: Run-TryBot+1

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Patch Set 1: TryBot-Result+1

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Patch Set 1: Code-Review-1

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This PR (HEAD: 7346d20) has been imported to Gerrit for code review.

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