title: dplyr rename error: contains unknown variables
body: <p>Very simple, renaming colnames with dplyr gives me an odd error.</p>
  
  <pre><code> library(dplyr)
  
   df &lt;- data.frame(var1=c("one","two","three"),var2=c(1,2,3)) 
  
   df &lt;- 
      df %&gt;% 
      rename(var1=are.letters, var2=are.numbers)
  
  Error: `are.letters`, `are.numbers` contains unknown variables
  </code></pre>
  
  <p>second try </p>
  
  <pre><code> df &lt;- rename(df, var1=are.letters, var2=are.numbers)
  
  Error: `are.letters`, `are.numbers` contains unknown variables
  </code></pre>
  
  <p>Wondering if quoting....</p>
  
  <pre><code>df &lt;- 
      df %&gt;% 
      rename('var1'='are.letters', 'var2'='are.numbers')
  
  Error: `are.letters`, `are.numbers` contains unknown variables
  </code></pre>