pubsub: use iterator.Done
pubsub: use iterator.Done

Change-Id: I7bb8d7f2d85372b09b90c61a9a03f76edc9820fa
Reviewed-on: https://code-review.googlesource.com/7438
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go