title: pandas replace zeros with previous non zero value
body: <p>I have the following dataframe:</p>
  
  <pre><code>index = range(14)
  data = [1, 0, 0, 2, 0, 4, 6, 8, 0, 0, 0, 0, 2, 1]
  df = pd.DataFrame(data=data, index=index, columns = ['A'])
  </code></pre>
  
  <p>How can I fill the zeros with the previous non-zero value using pandas? Is there a fillna that is not just for "NaN"?.  </p>
  
  <p>The output should look like:</p>
  
  <pre><code>[1, 1, 1, 2, 2, 4, 6, 8, 8, 8, 8, 8, 2, 1]
  </code></pre>
  
  <p>(This question was asked before here <a href="https://stackoverflow.com/questions/30488961/fill-zero-values-of-1d-numpy-array-with-last-non-zero-values">Fill zero values of 1d numpy array with last non-zero values</a> but he was asking exclusively for a numpy solution)</p>