x/oauth2/clientcredentials: context values are not passed to oauth2 requests that retrieve tokens

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

It does reproduce with the latest version of golang.org/x/oauth2. I guess this is an issue about this subrepository.

What operating system and processor architecture are you using (go env)?

What did you do?

https://play.golang.org/p/iOdgycJZA4f

This example requires facebook credentials, but the bug does reproduce with any credential source.

What did you expect to see?

There are two requests, one for retrieving an oauth2 token, and the other to get the actual URL.

I expected the two requests to see the same context values, which would generate an output like

context value
context value

What did you see instead?

<nil>
context value

The context value is not passed to the first request.