FakeClient should support switching origins

[langalex]

I'm writing a test where I register credentials on one subdomain (e.g. www.example.com) and then use them on another (auth.example.com), using a common rp id (example.com). This requires switching the origin in FakeClient, which can currently only be hacked via instance_variable_set.

Changing origin to an attr_accessor should fix the problem.