title: Using tidyr complete() with column names specified in variables
body: <p>I am having trouble using the <code>tidyr::complete()</code> function with column names as variables.</p>
  <p>The built-in example works as expected:</p>
  <pre><code>df &lt;- data_frame(
   group = c(1:2, 1),
   item_id = c(1:2, 2),
   item_name = c(&quot;a&quot;, &quot;b&quot;, &quot;b&quot;),
   value1 = 1:3,
   value2 = 4:6
  )
  
  df %&gt;% complete(group, nesting(item_id, item_name)) 
  </code></pre>
  <p>However, when I try to provide the column names as character strings, it produces an error.</p>
  <pre><code>gr=&quot;group&quot;
  id=&quot;item_id&quot;
  name=&quot;item_name&quot;
  df %&gt;% complete_(gr, nesting_(id, name),fill = list(NA))
  </code></pre>