title: Python pandas fillna only one row with specific value
body: <p>EDITED:</p>
  
  <p>I have (not a very simple) a dataframe:</p>
  
  <pre><code>df = pd.DataFrame([1, 2, np.nan, np.nan, np.nan, np.nan, 3, 4
      , np.nan, np.nan, np.nan, 5], columns=['att1'])
  
       att1
  0  1.0000
  1  2.0000
  2     nan
  3     nan
  4     nan
  5     nan
  6  3.0000
  7  4.0000
  8     nan
  9     nan
  10    nan
  11 5.0000
  </code></pre>
  
  <p>I want fill <code>NAN</code> values with the previous not <code>NAN</code> value except the last <code>NAN</code> value. I want the last <code>NAN</code> value to be <code>NAN</code> after filling. How can I do that?</p>
  
  <p>I want this result:</p>
  
  <pre><code>     att1
  0  1.0000
  1  2.0000
  2  2.0000
  3  2.0000
  4  2.0000
  5     nan
  6  3.0000
  7  4.0000
  8  4.0000
  9  4.0000
  10    nan
  11 5.0000
  </code></pre>
  
  <p>I tried this:</p>
  
  <pre><code>df = df.fillna(value='missing', method='bfill', limit=1)
  df = df.fillna(method='ffill')
  </code></pre>
  
  <p>But the first row gives this error:</p>
  
  <pre><code>ValueError: cannot specify both a fill method and value
  </code></pre>
  
  <p>Why there is this limitation in pandas 0.17.1 / Python 3.5?
  Thank you!</p>