title: kafka consumer seek is not working: AssertionError: Unassigned partition
body: <p>the kafka consumer <code>con</code> defined below works perfectly fine when I try to receive messages form my topic; however, it's giving me trouble when I try to change the offset using <code>seek</code> method or any of its variations. i.e. <code>seek_to_beginning</code>, <code>seek_to_end</code></p>
  
  <pre><code>from kafka import KafkaConsumer, TopicPartition
  
  con = KafkaConsumer(my_topic, bootstrap_servers = my_bootstrapservers, group_id = my_groupid)
  p = con.partitions_for_topic(my_topic)
  my_partition = p.pop()
  tp = TopicPartition(topic = my_topic, partition = my_partition)
  print ('*** tp: ', tp)
  con.seek_to_beginning(tp)
  </code></pre>
  
  <p>it generates the following output and the error below:</p>
  
  <pre><code>*** tp:  TopicPartition(topic='mytopic', partition=0)
  File "/myhome/anaconda3/lib/python3.6/site-packages/kafka/consumer/group.py", line 735, in seek_to_beginning
  assert p in self._subscription.assigned_partitions(), 'Unassigned partition'
  AssertionError: Unassigned partition
  </code></pre>
  
  <p>The 'Unassigned partition' error does not look valid to me because I just get the partition and the TopicPartition from the consumer itself and pass it back into the seek method so it is definitely assigned.
  any ideas?</p>