Set ID before invoking the client callback

Set ID before invoking the client callback

This change makes SignonIdentity process the results from
store_credentials() before invoking the client callback. This is
important because otherwise any SignonAuthSession created by the client
from within the callback would not be bound to the identity.

Also, add a unit test to catch any regressions.

Fixes: http://code.google.com/p/accounts-sso/issues/detail?id=132