title: separate in tidyr, error please supply column name
body: <p>The   dataset that is used in many R tutorials has an initial column with no header that lists the names of the cars, according to make and model. </p>
  
  <p>I was trying to separate the make and the model of car. </p>
  
  <p>First I made a column of the first, headerless column, called "names".</p>
  
  <pre><code>&gt; mtcars$names &lt;- rownames(mtcars)
  
  &gt; rownames(mtcars) &lt;- NULL
  </code></pre>
  
  <p>This gave me a column that was a character vector with the following results:</p>
  
  <pre><code>&gt; mtcars$names
  </code></pre>
  
  <p>With the following output:</p>
  
  <pre><code>[1] "Mazda RX4"           "Mazda RX4 Wag"    
  [3] "Datsun 710"          "Hornet 4 Drive"   
  </code></pre>
  
  <p>etc</p>
  
  <p>I then tried this:</p>
  
  <pre><code>separate(mtcars, colname = names, into = c('make', 'model')
  </code></pre>
  
  <p>and got the following error:</p>
  
  <p>Error: Please supply column name</p>