title: Filling missing dates in spark dataframe column
body: <p>I've a spark data frame with columns - "date" of type <code>timestamp</code> and "quantity" of type <code>long</code>. For each date, I've some value for quantity. The dates are sorted in increasing order. But there are some dates which are missing. 
  For eg - 
  Current df - </p>
  
  <pre><code>Date        |    Quantity
  10-09-2016  |    1
  11-09-2016  |    2
  14-09-2016  |    0
  16-09-2016  |    1
  17-09-2016  |    0
  20-09-2016  |    2
  </code></pre>
  
  <p>As you can see, the df has some missing dates like 12-09-2016, 13-09-2016 etc. I want to put 0 in the quantity field for those missing dates such that resultant df should look like - </p>
  
  <pre><code>Date        |    Quantity
  10-09-2016  |    1
  11-09-2016  |    2
  12-09-2016  |    0
  13-09-2016  |    0
  14-09-2016  |    0
  15-09-2016  |    0
  16-09-2016  |    1
  17-09-2016  |    0
  18-09-2016  |    0
  19-09-2016  |    0
  20-09-2016  |    2
  </code></pre>
  
  <p>Any help/suggestion regarding this will be appreciated. Thanks in advance.
  Note that I am coding in scala. </p>