Fetch messages for a pull subscription without blocking (none pending):

>>> from gcloud.pubsub import Topic
>>> from gcloud.pubsub import Subscription
>>> topic = Topic('topic_name')
>>> subscription = Subscription('subscription_name', topic)
- >>> received = subscription.pull(max_messages=1)  # API request
+ >>> received = subscription.pull(return_immediately=True)  # API request
>>> messages = [recv[1] for recv in received]
>>> [message.id for message in messages]
[]