title: combine rows in data frame containing NA to make complete row
body: <p>I know this is a duplicate Q but I can't seem to find the post again</p>
  
  <p>Using the following data</p>
  
  <pre><code>df &lt;- data.frame(A=c(1,1,2,2),B=c(NA,2,NA,4),C=c(3,NA,NA,5),D=c(NA,2,3,NA),E=c(5,NA,NA,4))
  
    A  B  C  D  E
    1 NA  3 NA  5
    1  2 NA  2 NA
    2 NA NA  3 NA
    2  4  5 NA  4
  </code></pre>
  
  <p>Grouping by <code>A</code>, I'd like the following output using a <code>tidyverse</code> solution</p>
  
  <pre><code>  A  B  C  D  E
    1  2  3  2  5
    2  4  5  3  4
  </code></pre>
  
  <p>I have many groups in <code>A</code>. I think I saw an answer using <code>coalesce</code> but am unsure how to get it work. I'd like a <strong>solution that works with <code>characters</code></strong> as well. Thanks!</p>