title: Recode a pandas.Series containing 0, 1, and NaN to False, True, and NaN
body: <p>Suppose I have a Series with NaNs:</p>
  
  <pre><code>pd.Series([0, 1, None, 1])
  </code></pre>
  
  <p>I want to transform this to be equal to:</p>
  
  <pre><code>pd.Series([False, True, None, True])
  </code></pre>
  
  <p>You'd think <code>x == 1</code> would suffice, but instead, this returns:</p>
  
  <pre><code>pd.Series([False, True, False, True])
  </code></pre>
  
  <p>where the null value has become <code>False</code>. This is because <code>np.nan == 1</code> returns <code>False</code>, rather than <code>None</code> or <code>np.nan</code> as in R.</p>
  
  <p>Is there a nice, vectorized way to get what I want?</p>