title: rowSums with all NA
body: <pre><code>df &lt;- data.frame(a = c(1, 1, NA, 0, 1, 0),
                   b = c(0, 1, NA, NA, 0, 1),
                   c = c(NA, 0, NA, 0, 1, NA),
                   d = c(1, NA, NA, 1, 1, 0))
  
  rowSums(df)
  #[1] NA NA NA NA  3 NA
  rowSums(df, na.rm=T)
  #[1] 2 2 0 1 3 1
  </code></pre>
  
  <p>The first I get, but my assumption and hope was that the third observation would return <code>NA</code>. Is there a way to get it to return NA for the third observation?</p>