title: Update values of pandas dataframe with corresponding value in another dataframe
body: <p>Suppose I have two dataframes:</p>
  
  <pre><code>df1
  
  A    B
  -------
  foo  2
  bar  3
  baz  4
  </code></pre>
  
  <p>and</p>
  
  <pre><code>df2
  
  A    B
  -------
  foo  40
  bar  50
  </code></pre>
  
  <p>And I want the final result to be </p>
  
  <pre><code>A    B
  -------
  foo  40
  bar  50
  baz  4
  </code></pre>
  
  <p>I can't figure out how to coax <code>pd.update</code>, or <code>pd.merge</code> into doing this.</p>