Handle expires_on in int format

[jhendrixMSFT]

Unmarshal the value into an interface{} and perform the proper
conversion depending on the underlying type.

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[jhendrixMSFT]

Fixes #696

[jhendrixMSFT]

Looking at the related issue, it looks to me that expires_on, at least in that example, isn't the number of seconds from now but probably from the Unix epoch. I need to take a closer look.

[chlowell]

Everywhere except App Service, expires_on is epoch seconds, either as a number or a string.

[jhendrixMSFT]

And App Service is as a time-stamp correct?

[jhendrixMSFT]

OK I did a little digging. Token.Expires() already treats ExpiresOn as Unix time. It does mean though that our handling of expires_on in date-time format is incorrect at present.

need updated review