Investigate novel pub/sub failure

[dhermes]

In https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/75334450, the merge build for #1060, we got an (flaky) error we have yet to see:

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ERROR: test_message_pull_mode_e2e (pubsub.TestPubsub)
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Traceback (most recent call last):
 File "/home/travis/build/GoogleCloudPlatform/gcloud-python/system_tests/pubsub.py", line 123, in test_message_pull_mode_e2e
 subscription.acknowledge(ack_ids)
 File "/home/travis/build/GoogleCloudPlatform/gcloud-python/.tox/system-tests3/lib/python3.4/site-packages/gcloud/pubsub/subscription.py", line 230, in acknowledge
 method='POST', path='%s:acknowledge' % (self.path,), data=data)
 File "/home/travis/build/GoogleCloudPlatform/gcloud-python/.tox/system-tests3/lib/python3.4/site-packages/gcloud/connection.py", line 419, in api_request
 error_info=method + ' ' + url)
gcloud.exceptions.BadRequest: 400 You have not specified an ack ID in the request. (POST https://pubsub.googleapis.com/v1/projects/precise-truck-742/subscriptions/subscribing-now:acknowledge)

[dhermes]

Closing since #1085 also has issue