x/oauth2: apply the expires_in value returned by the server to Token.ExpiresIn

[soh335]

In typical usage, Token.Expiry alone is sufficient. However, the ExpiresIn field was introduced in golang/go#61417. Even when a server returns an expires_in value in
methods like Config.Exchange, only the Expiry field is updated, leaving ExpiresIn unchanged, which can cause confusion.

This change ensures that the ExpiresIn field is properly updated when the server provides an expires_in value.

[gopherbot]

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Matt Hickford:

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

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Patch Set 1:

(1 comment)

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Patch Set 1: TryBot-Result+1

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[peick]

I would make it more clear here what "now" means. Indeed it's the time when the token was fetched.