OpenID module doesn't always properly clear the page cache

When page caching is turned on during automated test runs (see #606840-118: Enable internal page cache by default) some of the failures are in the following test cases: OpenIDFunctionalTestCase, OpenIDInvalidIdentifierTransitionTestCase

These may be due to some kind of bug where adding new OpenIDs doesn't clear the page cache. Needs more investigation to figure out exactly what's going on here.