Proposal: Populate Token.ExpiresIn when converting from internal.Token

[soh335]

The Token.ExpiresIn field was added in golang/go#61417. However, when retrieving a Token via methods like Config.Exchange, even though the server may return an expires_in value, only the Token.Expiry field is updated, and the Token.ExpiresIn field remains unset.

While Token.Expiry is generally sufficient, it seems confusing to have an ExpiresIn field present but left as a zero value. What do you think about populating the Token.ExpiresIn field when converting from internal.Token to Token?

#748

Alternatively, how about clarifying in the documentation for the ExpiresIn field that methods like Config.Exchange within the oauth2 package do not populate the Token.ExpiresIn field?

#747