title: how to resample without skipping nan values in pandas
body: <p>I am trying get the 10 days aggregate of my data which has NaN values. The sum of 10  days should return a nan values if there is a NaN value in the 10 day duration.</p>
  <p>When I apply the below code, pandas is considering NaN as Zero and returning the sum of remaining days.</p>
  <pre><code>dateRange = pd.date_range(start_date, periods=len(data), freq='D')
  # Creating a data frame so that the timeseries can handle numpy array.
  df = pd.DataFrame(data)
  base_Series = pd.DataFrame(list(df.values), index=dateRange)
  # Converting to aggregated series
  agg_series = base_Series.resample('10D', how='sum')
  agg_data = agg_series.values 
  </code></pre>
  <p><strong>Sample Data:</strong></p>
  <pre><code>2011-06-01  46.520536
  2011-06-02   8.988311
  2011-06-03   0.133823
  2011-06-04   0.274521
  2011-06-05   1.283360
  2011-06-06   2.556313
  2011-06-07   0.027461
  2011-06-08   0.001584
  2011-06-09   0.079193
  2011-06-10   2.389549
  2011-06-11        NaN
  2011-06-12   0.195844
  2011-06-13   0.058720
  2011-06-14   6.570925
  2011-06-15   0.015107
  2011-06-16   0.031066
  2011-06-17   0.073008
  2011-06-18   0.072198
  2011-06-19   0.044534
  2011-06-20   0.240080
  </code></pre>
  <p><strong>Output:</strong></p>
  <pre><code>2011-06-01  62.254651
  2011-06-11   7.301481
  </code></pre>