title: Row names disappear after as.matrix
body: <p>I notice that if the row names of the dataframe follows a sequence of numbers from 1 to the number of rows. The row names of the dataframe will disappear after using <code>as.matrix</code>. But the row names re-appear if the row name is not a sequence.</p>
  <p>Here are a reproducible example:</p>
  <pre><code>test &lt;- as.data.frame(list(x=c(0.1, 0.1, 1), y=c(0.1, 0.2, 0.3)))
  rownames(test)
  # [1] &quot;1&quot; &quot;2&quot; &quot;3&quot;
  
  rownames(as.matrix(test))
  # NULL
  
  rownames(as.matrix(test[c(1, 3), ]))
  # [1] &quot;1&quot; &quot;3&quot;
  </code></pre>
  <p>Does anyone have an idea on what is going on?</p>
  <p>Thanks a lot</p>