title: Replace NA´s in dates with another date
body: <p>Data:</p>
  
  <pre><code>DB1 &lt;- data.frame(orderItemID  = 1:10,     
  orderDate = c("2013-01-21","2013-03-31","2013-04-12","2013-06-01","2014-01-01", "2014-02-19","2014-02-27","2014-10-02","2014-10-31","2014-11-21"),  
  deliveryDate = c("2013-01-23", "2013-03-01", "NA", "2013-06-04", "2014-01-03", "NA", "2014-02-28", "2014-10-04", "2014-11-01", "2014-11-23"))
  </code></pre>
  
  <p>Expected Outcome: </p>
  
  <pre><code>   DB1 &lt;- data.frame(orderItemID  = 1:10,     
   orderDate= c("2013-01-21","2013-03-31","2013-04-12","2013-06-01","2014-01-01", "2014-02-19","2014-02-27","2014-10-02","2014-10-31","2014-11-21"),  
  deliveryDate = c("2013-01-23", "2013-03-01", "2013-04-14", "2013-06-04", "2014-01-03", "2014-02-21", "2014-02-28", "2014-10-04", "2014-11-01", "2014-11-23"))
  </code></pre>
  
  <p>My question is similar to another one I posted: so don´t be confused. 
  As you can see above I have some missing values in the delivery dates and I want to replace them by another date. That date should be the order date of the specific item + the average delivery time in (full) days.(2days)
  The average delivery time is the time calculated from the average value of all samples that do not contain Missing values = (2days+1day+3days+2days+1day+2days+1day+2days):8=1,75</p>
  
  <p>So I want to replace the NA in delivery time with the order date +2days. When there´s no NA, the date should stay the same.</p>
  
  <p>I tried this already (with <code>lubridate</code>), but it´s not working :( </p>
  
  <pre><code>DB1$deliveryDate[is.na(DB1$deliveryDate) ] &lt;- DB1$orderDate + days(2)
  </code></pre>
  
  <p>Can someone plz help me?</p>