title: column renaming with dplyr
body: <p>I am trying to rename a column using <code>dplyr::rename</code>, but failing.. </p>
  
  <pre><code>library(dplyr)
  
  cars %&gt;% 
    filter(speed &gt; 20) %&gt;% 
    rename(speed = new_speed)
  
  # Error: Unknown variables: new_speed.
  </code></pre>
  
  <p>Any ideas what's wrong there?</p>