title: Filter Pyspark dataframe column with None value
body: <p>I'm trying to filter a PySpark dataframe that has <code>None</code> as a row value:</p>
  
  <pre><code>df.select('dt_mvmt').distinct().collect()
  
  [Row(dt_mvmt=u'2016-03-27'),
   Row(dt_mvmt=u'2016-03-28'),
   Row(dt_mvmt=u'2016-03-29'),
   Row(dt_mvmt=None),
   Row(dt_mvmt=u'2016-03-30'),
   Row(dt_mvmt=u'2016-03-31')]
  </code></pre>
  
  <p>and I can filter correctly with an string value:</p>
  
  <pre><code>df[df.dt_mvmt == '2016-03-31']
  # some results here
  </code></pre>
  
  <p>but this fails:</p>
  
  <pre><code>df[df.dt_mvmt == None].count()
  0
  df[df.dt_mvmt != None].count()
  0
  </code></pre>
  
  <p>But there are definitely values on each category. What's going on?</p>