title: Allow grouping with NA in aggregate function
body: <p>Here is dummy data</p>
  
  <pre><code>temp.df &lt;- data.frame(count = rep(1,6), x = c(1,1,NA,NA,3,10), y=c("A","A","A","A","B","B"))
  </code></pre>
  
  <p>When I apply aggregate as given below:</p>
  
  <pre><code>aggregate(count ~ x + y, data=temp.df, FUN=sum, na.rm=FALSE, na.action=na.pass)
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>   x y count
  1  1 A     2
  2  3 B     1
  3 10 B     1
  </code></pre>
  
  <p>However, I would like the following output:</p>
  
  <pre><code>   x  y count
  1  NA A     2
  2  1  A     2
  3  3  B     1
  4 10  B     1
  </code></pre>
  
  <p>Hope it makes sense.Thanks in advance.</p>