Fix broken api module: use the string versions of the client secret

This is ugly, but because `Consumer#secret` is a UUID, we should be using
the `Consumer#client_secret` property here which is a string (unicode)
representation of the secret.

The prior state causes 500s on the token endpoint on a fresh checkout.
It's unlikely to be a problem, but it doesn't feel good to me :-D.
Can't the test just set it to None?