title: No grid lines in row header using grid.table() from gridExtra
body: <p>When I shade the row headers in my theme for grid.table, the horizontal lines between the rows don't appear. Sometimes one line will show, but it's thinner than the ones between the core cells. </p>
  
  <p>If I set the row padding to 5 lines, the gridlines sometimes appear, but that obviously isn't practical.</p>
  
  <p>Here's an example:</p>
  
  <pre><code>library(grid)
  library(gridExtra)
  t = ttheme_default(
      rowhead=list(
          fg_params=list(fontface="plain"),
          bg_params=list(fill=c(NA, rep("grey80",5)))
      )
  )
  grid.newpage()
  grid.table(iris[1:5, 1:3], theme=t)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/olPA7.png" alt="Example output"></p>
  
  <p>The answer below from baptiste solved my problem. Thanks!</p>