Don't include timestamp in renderTable by default

[wch]

By default, print.xtable includes comments with the R version, xtable version, and timestamp, like this:

<!-- html table generated in R 3.4.0 by xtable 1.8-2 package -->
<!-- Tue Jun 27 12:50:40 2017 -->
<table border=1>
<tr> <th>  </th> <th> temperature </th> <th> pressure </th>  </tr>
  <tr> <td align="right"> 1 </td> <td align="right"> 0.00 </td> <td align="right"> 0.00 </td> </tr>
...

When this is included in the output of renderTable, it results in spurious changes in for test results. This PR removes those comments, so the output is like this:

<table border=1>
<tr> <th>  </th> <th> temperature </th> <th> pressure </th>  </tr>
  <tr> <td align="right"> 1 </td> <td align="right"> 0.00 </td> <td align="right"> 0.00 </td> </tr>
...