title: dplyr to output class data.frame
body: <p>I can summarise a data frame with <code>dplyr</code> like this:</p>
  
  <pre><code>mtcars %&gt;%
  group_by(cyl) %&gt;%
  summarise(mean(mpg))
  </code></pre>
  
  <p>To convert the output back to class <code>data.frame</code>, my current approach is this:</p>
  
  <pre><code>as.data.frame(mtcars %&gt;%
  group_by(cyl) %&gt;%
  summarise(mean(mpg)))
  </code></pre>
  
  <p>Is there any way to get <code>dplyr</code> to output a class <code>data.frame</code> without having to use <code>as.data.frame</code>?</p>