title: Dataframe.rolling().mean not calculating moving average
body: <p>I've been trying to calculate a moving avarage using pandas, but when I use the Dataframe.rolling().mean(), it copies the value instead.</p>
  
  <pre><code>stock_info['stock'].head()
  </code></pre>
  
  <pre>
      Fecha               Open        High        Low         Close       Volume
  0   04-05-2007 00:00:00 234,4593    255,5703    234,3532    246,8906    6044574
  1   07-05-2007 00:00:00 246,8906    254,7023    247,855     252,1563    2953869
  2   08-05-2007 00:00:00 252,1562    250,7482    244,9617    250,1695    2007217
  3   09-05-2007 00:00:00 250,1695    249,7838    245,9261    248,3757    2329078
  4   10-05-2007 00:00:00 248,8194    248,9158    244,9617    245,6368    2138002
  </pre>
  
  <pre><code>stock_info['stock']['MA'] = stock_info['stock']['Close'].rolling(window=2).mean()
  </code></pre>
  
  <pre>
      Fecha               Open        High        Low         Close       Volume  MA
  0   04-05-2007 00:00:00 234,4593    255,5703    234,3532    246,8906    6044574 246,8906
  1   07-05-2007 00:00:00 246,8906    254,7023    247,855     252,1563    2953869 252,1563
  2   08-05-2007 00:00:00 252,1562    250,7482    244,9617    250,1695    2007217 250,1695
  3   09-05-2007 00:00:00 250,1695    249,7838    245,9261    248,3757    2329078 248,3757
  4   10-05-2007 00:00:00 248,8194    248,9158    244,9617    245,6368    2138002 245,6368
  </pre>