title: rowSums but keeping NA values
body: <p>I am pretty sure this is quite simple, but seem to have got stuck...I have two xts vectors that have been merged together, which contain numeric values and NAs.</p>
  
  <p>I would like to get the rowSums for each index period, but keeping the NA values.</p>
  
  <p>Below is a reproducible example</p>
  
  <pre><code>set.seed(120)
  dd &lt;- xts(rnorm(100),Sys.Date()-c(100:1))
  dd1 &lt;- ifelse(dd&lt;(-0.5),dd*-1,NA)
  dd2 &lt;- ifelse((dd^2)&gt;0.5,dd,NA)
  mm &lt;- merge(dd1,dd2)
  mm$m &lt;- rowSums(mm,na.rm=TRUE)
  tail(mm,10)
  
                   dd1        dd2        m
  2013-08-02        NA         NA 0.000000
  2013-08-03        NA         NA 0.000000
  2013-08-04        NA         NA 0.000000
  2013-08-05 1.2542692 -1.2542692 0.000000
  2013-08-06        NA  1.3325804 1.332580
  2013-08-07        NA  0.7726740 0.772674
  2013-08-08 0.8158402 -0.8158402 0.000000
  2013-08-09        NA  1.2292919 1.229292
  2013-08-10        NA         NA 0.000000
  2013-08-11        NA  0.9334900 0.933490
  </code></pre>
  
  <p>In the above example on the 10th Aug 2013 I was hoping it would say <code>NA</code> instead of <code>0</code>, the same goes for the 2nd-4th Aug 2013.</p>
  
  <p>Any suggestions for an elegant way of getting NAs in the relevant places?</p>