title: Join common keys in hash one-liner
body: <p>I have this array of pairs:</p>
  
  <pre><code>[{"a"=&gt;"1"}, {"b"=&gt;"2"}, {"a"=&gt;"3"}, {"b"=&gt;"4"}, {"a"=&gt;"5"}]
  </code></pre>
  
  <p>I would like a method to merge the keys in common with multiple values to:</p>
  
  <pre><code>[{"a"=&gt;["1","3","5"]}, {"b"=&gt;["2","4"]}]
  </code></pre>