title: ReporteRs Character NAs Blank
body: <p>I would like to make a FlexTable that makes NA values appear as NA. When the column is of class numeric, I am able to do this. When the column is of class character, the cell appears empty. Is there a way to get my NA values to appear when they are part of a column of class character? I am aware that I could simply replace them with "NA", but I would strongly prefer not to do this.</p>
  
  <pre><code>library(ReporteRs)
  
  y &lt;- data.frame(var.1 = as.character(letters[1:5]), 
                  var.2 = as.numeric(1:5), 
                  stringsAsFactors = FALSE)
  y[3, 1] &lt;- NA
  y[4, 2] &lt;- NA
  
  FlexTable(y)
  </code></pre>