Stub out Subscriber in doctest
Stub out Subscriber in doctest

Now that we have fixed the threading bug we need to stub out these methods.
Update mocks to use streaming calls, for when these method stubs are removed.

landrito/google-cloud-ruby,landrito/google-cloud-ruby,landrito/google-cloud-ruby,quartzmo/gcloud-ruby,quartzmo/google-cloud-ruby,quartzmo/google-cloud-ruby,CloudVLab/google-cloud-ruby,CloudVLab/google-cloud-ruby,googleapis/google-cloud-ruby,landrito/google-cloud-ruby,CloudVLab/google-cloud-ruby,blowmage/gcloud-ruby,CloudVLab/google-cloud-ruby,googleapis/google-cloud-ruby,googleapis/google-cloud-ruby,quartzmo/google-cloud-ruby,dazuma/google-cloud-ruby,dazuma/google-cloud-ruby,googleapis/google-cloud-ruby,quartzmo/google-cloud-ruby,dazuma/google-cloud-ruby,googleapis/google-cloud-ruby,quartzmo/google-cloud-ruby,CloudVLab/google-cloud-ruby,dazuma/google-cloud-ruby,landrito/google-cloud-ruby,dazuma/google-cloud-ruby,dazuma/google-cloud-ruby,googleapis/google-cloud-ruby,quartzmo/google-cloud-ruby