title: pandas.DataFrame.rolling not working with huge floats
body: <p>I have an error with pandas rolling when using floats close to infinity. I display an example here:</p>
  
  <pre><code>import pandas as pd
  series = pd.Series(1.,index = pd.date_range('2015-01-01', periods=6))
  series[series.index[2]] = 1e19
  series
  2015-01-01    1.000000e+00
  2015-01-02    1.000000e+00
  2015-01-03    1.000000e+19
  2015-01-04    1.000000e+00
  2015-01-05    1.000000e+00
  2015-01-06    1.000000e+00
  Freq: D, dtype: float64
  series.rolling('2D', closed = 'left').mean()
  2015-01-01             NaN
  2015-01-02    1.000000e+00
  2015-01-03    1.000000e+00
  2015-01-04    5.000000e+18
  2015-01-05    5.000000e+18
  2015-01-06    5.000000e-01
  Freq: D, dtype: float64
  </code></pre>
  
  <p>The answer in the last bit should be 1! but it is 0.5. Why does rolling go nuts when using big numbers? same example with smaller float:</p>
  
  <pre><code>series[series.index[2]] = 1e9
  series.rolling('2D', closed = 'left').mean()
  2015-01-01            NaN
  2015-01-02            1.0
  2015-01-03            1.0
  2015-01-04    500000000.5
  2015-01-05    500000000.5
  2015-01-06            1.0
  Freq: D, dtype: float64
  </code></pre>