Discard possibly expired token before re-authenticating

Fixes bug 1192656

Previously, the attempt to re-authenticate on possible token
expiry actually re-used the expired token, which was clearly
bound to fail in the expired case.

Now the old authentication state is discarded before attempting
re-authentication.

Change-Id: I3fd125702061f7ac84eb501d2a488aab5b2385b9