Add Transport.Expiry() to query token expiration

[wwade]

Since it could be interesting in some cases, return this for the user to examine.

Also factor out the "is a token expired" logic and return the current value of "we think it's expired" when calling Expiry().

expiresAt, expired, err := trans.Expiry()

[bradleyfalzon]

@wwade thanks, and thanks for maintaining some consistency there too.

@wlynch thoughts, this looks good to me, makes sense for debug code to see what behaviour we're expecting. Existing tests should cover the additional code, would be nice to see a test on the new public method though?

[wwade]

@wlynch thoughts, this looks good to me, makes sense for debug code to see what behaviour we're expecting. Existing tests should cover the additional code, would be nice to see a test on the new public method though?

I threw in a quick test case to cover the branches 😄

[wlynch]

No other comments, so submitting.

Thanks! 🎉