title: In R, "Merge" seems to perform intersection, I need a Union of data frames
body: <p>I have two data frames:</p>
  
  <pre><code>&gt; cDates
  # A tibble: 4 x 2
      week ClosedPR
    &lt;date&gt;    &lt;int&gt;
  1 2017-01-09        1
  2 2017-02-06        1
  3 2017-05-22        1
  4         NA       72
  &gt; oDates
  # A tibble: 22 x 2
       week OpenPR
     &lt;date&gt;  &lt;int&gt;
  1 2016-09-05      1
  2 2016-09-12      1
  3 2016-10-10      2
  4 2016-12-19      1
  5 2017-01-02      1
  6 2017-01-09      1
  7 2017-01-16      1
  8 2017-01-23      2
  9 2017-02-20      3
  10 2017-03-06      2
  # ... with 12 more rows
  </code></pre>
  
  <p>When I perform a "merge", I get the intersection of the data:</p>
  
  <pre><code>        week OpenPR ClosedPR
  1 2017-01-09      1        1
  2 2017-05-22      3        1
  3       &lt;NA&gt;     27       72
  </code></pre>
  
  <p>What I'd like is the union of <em>all</em> the data not just the entries with common dates in "week".
  How can I do this?</p>