http2: check if transport allows h2c before determining if a request has been sent

[WeidiDeng]

Currently it's not possible to disable keepalive for h2c requests, as it will cause requests to fail.

Fixes golang/go#67671

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 31c2899) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/net/+/588635.

Important tips:

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[gopherbot]

Message from Weidi Deng:

Patch Set 3:

(1 comment)

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[gopherbot]

Message from Damien Neil:

Patch Set 3:

(1 comment)

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[gopherbot]

Message from Weidi Deng:

Patch Set 3:

(1 comment)

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