title: Pandas .resample() or .asfreq() fill forward times
body: <p>I'm trying to resample a dataframe with a time series from 1-hour increments to 15-minute.  Both <code>.resample()</code> and <code>.asfreq()</code> do almost exactly what I want, but I'm having a hard time filling the last three intervals.  </p>
  
  <p>I could add an extra hour at the end, resample, and then drop that last hour, but it feels hacky.</p>
  
  <p>Current code:</p>
  
  <pre><code>df = pd.DataFrame({'date':pd.date_range('2018-01-01 00:00', '2018-01-01 01:00', freq = '1H'), 'num':5})
  df = df.set_index('date').asfreq('15T', method = 'ffill', how = 'end').reset_index()
  </code></pre>
  
  <p>Current output:</p>
  
  <pre><code>                 date  num
  0 2018-01-01 00:00:00    5
  1 2018-01-01 00:15:00    5
  2 2018-01-01 00:30:00    5
  3 2018-01-01 00:45:00    5
  4 2018-01-01 01:00:00    5
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>                 date  num
  0 2018-01-01 00:00:00    5
  1 2018-01-01 00:15:00    5
  2 2018-01-01 00:30:00    5
  3 2018-01-01 00:45:00    5
  4 2018-01-01 01:00:00    5
  5 2018-01-01 01:15:00    5
  6 2018-01-01 01:30:00    5
  7 2018-01-01 01:45:00    5
  </code></pre>
  
  <p>Thoughts?</p>