title: rollapply na.rm = TRUE giving 0 values instead of NA's
body: <p>I have a simple question, that I can't seem to find the answer to on google, stackoverflow or stackexchange.  I am currently working with the example of <code>rollapply</code> to find the sum of some values that contain NA's.  For example:</p>
  
  <pre><code> z &lt;- zoo(c(NA, NA, NA, NA,2, 3, 4, 5, NA))
   rollapply(z, 3, sum, na.rm = TRUE, align = "right")
  </code></pre>
  
  <p>This outputs:</p>
  
  <pre><code>  3  4  5  6  7  8  9 
    0  0  2  5  9 12  9 
  </code></pre>
  
  <p>This looks good, however, there are two times where there are 3 NA's in a row.  The sum feature exchanges NA's to 0's.  Unfortunately, that won't work with the data I'm going to work with since 0 is a meaningful value.  Is there a way to replace the 0's with NA's again?</p>
  
  <p>I'm looking for an output as below:</p>
  
  <pre><code>  3   4   5  6  7  8  9 
    NA  NA  2  5  9 12  9 
  </code></pre>
  
  <p>Thank you in advance!</p>