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.
You have to set `request.client`, as well, in the test. To me, this is the same. These are prerequisites of the call that the framework is supposed to have taken care of.