The "expires_at" attribute should also be recalculated on refresh

[chutzimir]

By nil-ing the expires_at attribute, next time expires_at is accessed
through the reader, it will get recalculated.

As it is currently, when refreshing the access_token, the issued_at,
expires_in would get updated, but expires_at may still indicate that the
token has expired.

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[chutzimir]

Done, @googlebot,

[googlebot]

CLAs look good, thanks!

[inopinatus]

I have spent two hours today on "why is it writing a new access token to my credential store on every freaking API call?!"

Now I know why.
Please merge this PR and release.