title: pandas resample interpolate is producing NaNs
body: <p>Modified from <a href="https://stackoverflow.com/a/39730730/125507">this example</a>:</p>
  
  <pre><code>import io
  import pandas as pd
  import matplotlib.pyplot as plt
  
  data = io.StringIO('''\
  Values
  1992-08-27 07:46:48,1
  1992-08-27 08:00:48,2
  1992-08-27 08:33:48,4
  1992-08-27 08:43:48,3
  1992-08-27 08:48:48,1
  1992-08-27 08:51:48,5
  1992-08-27 08:53:48,4
  1992-08-27 08:56:48,2
  1992-08-27 09:03:48,1
  ''')
  s = pd.read_csv(data, squeeze=True)
  s.index = pd.to_datetime(s.index)
  
  res = s.resample('4s').interpolate('linear')
  print(res)
  plt.plot(res, '.-')
  plt.plot(s, 'o')
  plt.grid(True)
  </code></pre>
  
  <p>It works as expected:</p>
  
  <pre><code>1992-08-27 07:46:48    1.000000
  1992-08-27 07:46:52    1.004762
  1992-08-27 07:46:56    1.009524
  1992-08-27 07:47:00    1.014286
  1992-08-27 07:47:04    1.019048
  1992-08-27 07:47:08    1.023810
  1992-08-27 07:47:12    1.028571
  ....
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/tlkcz.png" rel="noreferrer"><img src="https://i.stack.imgur.com/tlkcz.png" alt="interpolated values"></a></p>
  
  <p>but if I change the resample to <code>'5s'</code>, it produces only NaNs:</p>
  
  <pre><code>1992-08-27 07:46:45   NaN
  1992-08-27 07:46:50   NaN
  1992-08-27 07:46:55   NaN
  1992-08-27 07:47:00   NaN
  1992-08-27 07:47:05   NaN
  1992-08-27 07:47:10   NaN
  1992-08-27 07:47:15   NaN
  ....
  </code></pre>
  
  <p>Why?</p>