title: GMail api users().history().list returns 'nextPageToken', but no 'history' field?
body: <p>I use the GMail API's <a href="https://developers.google.com/gmail/api/v1/reference/users/history/list" rel="nofollow noreferrer"><code>history.list</code></a> to retrieve a list of changed messages, this works fine with several pages of history - but sometimes when a <code>nextPageToken</code> is returned, it is used to retrieve the next page, which is returned <strong>without</strong> a <code>history</code> field. No <code>HttpError</code> is raised.</p>
  
  <pre><code>results = self.service.users ().history ().list (userId = self.account, startHistoryId = start).execute ()
  if 'history' in results:
    yield results['history']
  
  while 'nextPageToken' in results:
    pt = results['nextPageToken']
    results = self.service.users ().history ().list (userId = self.account, startHistoryId = start, pageToken = pt).execute ()
    yield results['history'] # this fails with missing 'history' member.
  </code></pre>