title: pandas replace NaN with NaT
body: <p>I have a series with some NaNs that I need to replace with NaTs. How can I do this?</p>
  
  <p>Here's a simple example with what I've tried so far:</p>
  
  <pre><code>&gt;&gt;&gt; s = pd.Series([np.NaN, np.NaN])
  &gt;&gt;&gt; s.fillna(pd.NaT)
  0   NaN
  1   NaN
  dtype: float64
  &gt;&gt;&gt; s.replace(np.NaN, pd.NaT)
  0   NaN
  1   NaN
  dtype: float64
  &gt;&gt;&gt; s.where(pd.notnull(s), pd.NaT)
  0    NaN
  1    NaN
  dtype: object
  </code></pre>
  
  <p>pandas version: 0.16.2</p>
  
  <p>numpy version: 1.9.2</p>
  
  <p>python version: 2.7.10</p>