jwt: assertion time is not updated before JWT refresh
jwt: assertion time is not updated before JWT refresh

The assertion and expiration time in the claim set do not get updated
before fetching a new JWT, hence all refreshed tokens will already be
expired.

LGTM=adg
R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/61300043

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

desireebisco/goauth2,combucha/goauth2,lvzhihao/goauth2,happydpc/goauth2,jeisenberg/goauth2,smagic39/goauth2,djacobow/goauth2,avisri/goauth2,mattga/goauth2,skyeunknown084/goauth2,imrickrick/goauth2,stanwu/goauth2,badu/goauth2,MrDangerous/goauth2,pkieltyka/goauth2,odeke-em/goauth2,treetopllc/goauth2,epelc/goauth2,udhos/goauth2,aquarat/goauth2,hiteshjoshi/goauth2