title: pandas reindex fill in missing dates
body: <p>I have a dataframe with an index of dates. Each data is the first of the month. I want to fill in all missing dates in the index at a daily level.</p>
  
  <p>I thought this should work:</p>
  
  <pre><code>daily=pd.date_range('2016-01-01', '2018-01-01', freq='D')
  df=df.reindex(daily)
  </code></pre>
  
  <p>But it's returning <code>NA</code> in rows that should have data in (1st of the month dates) Can anyone see the issue?</p>