title: Pandas: fill in NaN values with dictionary references another column
body: <p>I have a dictionary that looks like this</p>
  
  <p><code>dict = {'b' : '5', 'c' : '4'}</code></p>
  
  <p>My dataframe looks something like this</p>
  
  <pre><code>   A  B
  0  a  2
  1  b  NaN
  2  c  NaN
  </code></pre>
  
  <p>Is there a way to fill in the NaN values using the dictionary mapping from columns A to B while keeping the rest of the column values?</p>