Allow credential without explicit expire time

[galacticvac]

Certain sites provide an access token without an expiration. Currently this causes a NullPointerException. This change is to allow valid credentials that don't have an expiry.

[googlebot]

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[galacticvac]

I signed it!

…

On Sat, Feb 4, 2017 at 1:43 PM, googlebot ***@***.***> wrote: Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). 📝 *Please visit https://cla.developers.google.com/ <https://cla.developers.google.com/> to sign.* Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks. ------------------------------ - If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data <https://cla.developers.google.com/clas> and verify that your email is set on your git commits <https://help.github.com/articles/setting-your-email-in-git/>. - If you signed the CLA as a corporation, please let us know the company's name. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#144 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AYRKqjU12J0sYdCSOOBaHSWFNmcAjmTXks5rZMbYgaJpZM4L3PN_> .

[googlebot]

CLAs look good, thanks!

[tswast]

@garrettjonesgoogle PTAL (or is someone else maintaining this?)