title: Replace null in a column of a dataframe with other value
body: <p>I have a <code>Dataset</code> like the following</p>
  
  <pre><code>monthYear    code
  201601       11
  201601       12
  201601       12
  201601       10
  201602       null
  201602       21
  201602       21
  201602       21
  201603       null
  </code></pre>
  
  <p>When <code>code</code> is <code>null</code> I want to replace that with the <code>code</code> that appeared the most during the last month. For the above example, the first <code>null</code> will get replaced by <code>12</code> and the second one with <code>21</code>.</p>
  
  <p>So the result would be the following.</p>
  
  <pre><code>monthYear    code
  201601       11
  201601       12
  201601       12
  201601       10
  201602       12
  201602       21
  201602       21
  201602       21
  201603       21
  </code></pre>
  
  <p>How can I achieve this?</p>