`topic.exists` method does not resolve promise

[hansman]

Running @google-cloud/pubsub@0.7.1, the topic.exists does not get resolved but is pending indefinitely. This is not in @google-cloud/pubsub@0.6.2.

[jmdobry]

Can you please move this issue to https://github.com/GoogleCloudPlatform/google-cloud-node?

[hansman]

googleapis/google-cloud-node#1957