Fix issues with Google Apps domains

1. Change to the logic for falling back to a proxy in the _resolveHostMeta function. Now works with Google Apps domains that returns an unexpected document with no providers in it.
2. Change to the _verifyAssertionAgainstProvider function. Now trims the discovered OpenID provider endpoint and the provider endpoint in assertion response in the same way. Fixes an issues with Google Apps domains. (Could fix the issue by trimming neither but I assume the trim was previously introduced for a reason.)