title: graphlab create sframe how to get SArray median
body: <p>I'm studying graphlab create
  with </p>
  
  <pre><code>data=graphlab.SFrame.read_csv('test.csv')
  </code></pre>
  
  <p>im trying to get median of one of columns</p>
  
  <pre><code>data_train.fillna(('Credit_History',data_train['Credit_History'].median()))
  </code></pre>
  
  <p>but I got error</p>
  
  <pre><code>---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-247-50ed3eb09dcc&gt; in &lt;module&gt;()
  ----&gt; 1 data_train.fillna(('Credit_History',data_train['Credit_History'].median()))
  
  AttributeError: 'SArray' object has no attribute 'median'
  </code></pre>
  
  <p>data.show() will show median of this column though
  anyone knows how to fix this? </p>