title: R "unnesting" dataframe results in an error
body: <p>I have a dataset that looks like this -</p>
  
  <pre><code>dataset = data.frame(Site=c(rep('A',6),rep('B',6)),
                       Date=c(rep(c('2019-05-31','2019-04-30','2019-03-31'),4)),
                       Question=c(rep('Q1',3),rep('Q2',3)),
                       Score=runif(12,0.5,1),
                       Average=runif(12,0.5,1))
  </code></pre>
  
  <p>My objective is to spread the the <code>Score</code> and <code>Average</code> columns based on the <code>Date</code> column.</p>
  
  <p>Using <code>tidyverse</code>, I manipulate the data -</p>
  
  <pre><code>library(tidyverse)
  dataset %&gt;% 
    nest(Score, Average, .key = 'value_col') %&gt;% 
    spread(key = Date, value = value_col) %&gt;% 
    unnest(.preserve = c("Site", "Question"), .sep = "_")
  </code></pre>
  
  <p>And this results in the final dataframe I am looking for -</p>
  
  <pre><code>  Site Question 2019-03-31_Score 2019-03-31_Average 2019-04-30_Score 2019-04-30_Average 2019-05-31_Score 2019-05-31_Average
  1    A       Q1        0.5070755          0.6948877        0.8046608          0.8359777        0.7653232          0.5259696
  2    A       Q2        0.5255425          0.9482262        0.9796590          0.7612117        0.9819698          0.7710665
  3    B       Q1        0.6963277          0.5416473        0.7753426          0.6710344        0.8219699          0.5310356
  4    B       Q2        0.9993356          0.6293783        0.8125886          0.5007390        0.6385580          0.5238838
  </code></pre>
  
  <p>However when I add a <strong>new site</strong> to the original dataframe...</p>
  
  <pre><code>new_site= data.frame(Site=c(rep('C',4)),
                       Date=c('2019-05-31','2019-03-31','2019-05-31','2019-03-31'),
                       Question=c(rep('Q1',2),rep('Q2',2)),
                       Score=runif(4,0.5,1),
                       Average=runif(4,0.5,1))
  
  new_dataset = rbind(dataset,new_site)
  </code></pre>
  
  <p>and re-run the data manipulation on the new dataset, I get the following error...</p>
  
  <pre><code>library(tidyverse)
  new_dataset %&gt;% 
    nest(Score, Average, .key = 'value_col') %&gt;% 
    spread(key = Date, value = value_col) %&gt;% 
    unnest(.preserve = c("Site", "Question"), .sep = "_")
  </code></pre>
  
  <blockquote>
    <p>Error: All nested columns must have the same number of elements.</p>
  </blockquote>
  
  <p>I figured that this is because the new site has <strong>one day of no data.</strong></p>
  
  <p>I'd like to know whether there's an alternate approach to treating this new dataset, and reaching the same format of output.</p>