title: pandas describe 0.18.0 vs pandas describe 0.17.0
body: <p>On one environment, I have pandas version 0.17.0 with numpy version 1.10.1.
  On another environment, I have pandas version 0.18.1 with numpy version 1.10.4.</p>
  
  <p>I run this piece of code </p>
  
  <pre><code>from pandas import Series
  import numpy as np
  Series([1,2,3,4,5,np.NaN]).describe()
  </code></pre>
  
  <p>With pandas version 0.17.0 I get this output: </p>
  
  <pre><code>count    5.000000 
  mean     3.000000
  std      1.581139
  min      1.000000
  25%      2.000000
  50%      3.000000
  75%      4.000000
  max      5.000000
  dtype: float64
  </code></pre>
  
  <p>with pandas version 0.18.1 i get this output: </p>
  
  <pre><code>count    5.000000
  mean     3.000000
  std      1.581139
  min      1.000000
  25%           NaN
  50%           NaN
  75%           NaN
  max      5.000000
  dtype: float64
  </code></pre>
  
  <p>what gives? </p>