title: Google Cloud DataFlow PubSubIO doesn't read from a full topic
body: <p>I'm trying to run a pipeline in Google Cloud DataFlow, in "Streaming" mode.
  The pipeline should read from a PubSub topic, however it doesn't actually read from the topic until I delete it, re-create it and re-publish all the messages to the topic AFTER the pipeline started.</p>
  
  <p>Is there any way to make the pipeline read already-published messages?</p>