title: Acknowledging pubsub messages through python synchronous pull does not work
body: <p>With the python <code>google-cloud-pubsub</code> library, acknowledging messages through the <code>subscriber.acknowledge()</code> does not acknowledge my messages. My ack deadline is set at 30 seconds. </p>
  
  <p>Here is my code: </p>
  
  <pre><code>from google.cloud import pubsub_v1
  
  project_id = "$$$$"
  subscription_name = "$$$$" 
  subscriber = pubsub_v1.SubscriberClient()
  subscription_path = subscriber.subscription_path(project_id, subscription_name)
  
  response = subscriber.pull(subscription_path, max_messages=10, timeout=15)
  
  for msg in response.received_messages:
      subscriber.acknowledge(subscription=subscription_path, ack_ids=[msg.ack_id])
  
  </code></pre>
  
  <p>Using <code>google-cloud-pubsub==1.0.2</code></p>
  
  <p>Any idea of what I'm doing wrong?</p>