title: pymongo update not working
body: <p>I'm trying to update a row in a mongoDB using pymongo. It seems to all work, except that it doesn't actually update the row! Is there something I'm missing (like a flush command?). Code snippet below:</p>
  
  <pre><code>In [161]: articles = connection.journals.articles.find()
  In [162]: articles[0]['_id']
  Out[162]: ObjectId('4ee61fc0df04c08c5c510b51')
  
  In [163]: articles[0]['newfield'] = 'Test'
  
  In [164]: connection.journals.articles.update({'_id': articles[0]['_id']}, articles[0], safe=True)
  Out[164]: 
  {u'connectionId': 62,
   u'err': None,
   u'n': 1,
   u'ok': 1.0,
   u'updatedExisting': True}
  
  In [165]: articles = connection.journals.articles.find()
  
  In [166]: articles[0]['_id']
  Out[166]: ObjectId('4ee61fc0df04c08c5c510b51')
  
  In [167]: articles[0]['newfield']
  ---------------------------------------------------------------------------
  KeyError                                  Traceback (most recent call last)
  
  script.py in &lt;module&gt;()
  ----&gt; 1 
        2 
        3 
        4 
        5 
  
  KeyError: 'newfield'
  </code></pre>