http2/h2c: Respect the req.Context()

[LukeShu]

When using h2c.NewHandler, the *http.Request object for h2c requests has a
.Context() that doesn't inherit from the *http.Server's BaseContext. This
is surprising for users of vanilla net/http, and is surprising to users of
http2.ConfigureServer; HTTP/1 requests inherit from that BaseContext, and
TLS h2 requests inherit from that BaseContext, but cleartext h2c requests
don't.

So, modify h2c.NewHander to respect that base Context, by way of the
hijacked Request's .Context().

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[LukeShu]

@googlebot I signed it!

[google-cla]

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[LukeShu]

@googlebot I signed it!

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[gopherbot]

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Patch Set 3: Code-Review+1

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Patch Set 3: Run-TryBot+1 Code-Review+2 Trust+1

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Message from Go Bot:

Patch Set 3:

Build is still in progress...
This change failed on linux-amd64-race:
See https://storage.googleapis.com/go-build-log/0f66fb7b/linux-amd64-race_4e90b1c5.log

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Patch Set 3: TryBot-Result-1

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Patch Set 3: Trust+1

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This PR is being closed because golang.org/cl/278972 has been merged.