title: get the last modified date of tables using bigquery tables GET api
body: <p>I am trying to get the list of tables and their last_modified_date using bigquery REST API.<br>
  In the bigquery API explorer I am getting all the fields correctly but when I use the api from Python code its returning 'None' for modified date.
  <br>
  This is the code written for the same in python</p>
  
  <pre><code>from google.cloud import bigquery
  client = bigquery.Client(project='temp')
  datasets = list(client.list_datasets())
  
  for dataset in datasets:
      print dataset.dataset_id
  
  for dataset in datasets:
      for table in dataset.list_tables():
          print table.table_id
          print table.created
          print table.modified
  </code></pre>
  
  <p>In this code I am getting created date correctly but modified date is 'None' for all the tables.</p>