net/http: support TCP half-close when HTTP is upgraded in ReverseProxy

[callthingsoff]

Rather than closing all sockets immediately when the one side gets an EOF, this CL closes the write stream on the socket to inform the transfer is finished.

Fixes #35892

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Message from Jes Cok:

Patch Set 1: Commit-Queue+1

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Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-02-15T13:46:56Z","revision":"17f250ed43bdc737df15effb17afe1b696e5352d"}

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Message from Jes Cok:

Patch Set 1: -Commit-Queue

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Patch Set 1:

This CL has passed the run

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Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result+1

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Message from Jes Cok:

Patch Set 1:

(1 comment)

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Patch Set 1:

(1 comment)

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Patch Set 1:

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Patch Set 1:

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