title: Pandas resample and ffill leaves NaN at the end
body: <p>I want to up-sample a series from weekly to daily frequency by forward filling the result.</p>
  
  <p>If the last observation of my original series is <code>NaN</code>, I would have expected this value to be replaced by the previous valid value, but instead it remains as <code>NaN</code>. </p>
  
  <p><strong>SETUP</strong></p>
  
  <pre><code>import numpy as np
  import pandas as pd
  
  all_dates = pd.date_range(start='2018-01-01', freq='W-WED', periods=4)
  
  ts = pd.Series([1, 2, 3], index=all_dates[:3])
  ts[all_dates[3]] = np.nan
  
  ts
  Out[16]: 
  2018-01-03    1.0
  2018-01-10    2.0
  2018-01-17    3.0
  2018-01-24    NaN
  Freq: W-WED, dtype: float64
  </code></pre>
  
  <p><strong>RESULT</strong></p>
  
  <pre><code>ts.resample('B').ffill() 
  
  ts.resample('B').ffill()
  Out[17]: 
  2018-01-03    1.0
  2018-01-04    1.0
  2018-01-05    1.0
  2018-01-08    1.0
  2018-01-09    1.0
  2018-01-10    2.0
  2018-01-11    2.0
  2018-01-12    2.0
  2018-01-15    2.0
  2018-01-16    2.0
  2018-01-17    3.0
  2018-01-18    3.0
  2018-01-19    3.0
  2018-01-22    3.0
  2018-01-23    3.0
  2018-01-24    NaN
  Freq: B, dtype: float64
  </code></pre>
  
  <p>While I was expecting the last value to be 3 as well.</p>
  
  <p>Does anyone has an <em>explanation</em> of this behaviour?</p>