title: r-convert list column into character vector where lists are characters
body: <p>I'm trying to convert a list into a single character value, or basically go from this:</p>
  
  <pre><code>test &lt;- data.frame(a = c(1,1,1,2,2,2), b = c("a", "b", "c", "d", "e", "f" )) %&gt;% 
    group_by(a) %&gt;% summarise(b = list(b))
  </code></pre>
  
  <p>to this:</p>
  
  <pre><code>test &lt;- data.frame(a = c(1,2), b = c("a, b, c", "d, e, f" )) 
  </code></pre>