title: Ignoring NaNs with str.contains
body: <p>I want to find rows that contain a string, like so:</p>
  
  <pre><code>DF[DF.col.str.contains("foo")]
  </code></pre>
  
  <p>However, this fails because some elements are NaN:</p>
  
  <blockquote>
    <p>ValueError: cannot index with vector containing NA / NaN values</p>
  </blockquote>
  
  <p>So I resort to the obfuscated</p>
  
  <pre><code>DF[DF.col.notnull()][DF.col.dropna().str.contains("foo")]
  </code></pre>
  
  <p>Is there a better way?</p>