title: How to unnest column-list?
body: <p>I have a tibble like:</p>
  
  <pre><code>tibble(a = c('first', 'second'), 
         b = list(c('colA' = 1, 'colC' = 2), c('colA'= 3, 'colB'=2))) 
  
  # A tibble: 2 x 2
    a      b        
    &lt;chr&gt;  &lt;list&gt;   
  1 first  &lt;dbl [2]&gt;
  2 second &lt;dbl [2]&gt;
  </code></pre>
  
  <p>Which a would like to turn into this form:</p>
  
  <pre><code># A tibble: 2 x 4
    a       colA  colB  colC
    &lt;chr&gt;  &lt;dbl&gt; &lt;dbl&gt; &lt;dbl&gt;
  1 first     1.   NA     2.
  2 second    3.    2.   NA 
  </code></pre>
  
  <p>I tried to use <code>unnest()</code>, but I am having issues preserving the elements' names from the nested values. </p>