title: How to merge two hashes with no new keys
body: <p>How could I merge two hashes that results in no new keys, meaning the merge would merge keys that exist in both hashes?</p>
  
  <p>For example, I want the following:</p>
  
  <pre><code>h = {:foo =&gt; "bar"}
  j = {:foo =&gt; "baz", :extra =&gt; "value"}
  
  puts h.merge(j)    # {:foo =&gt; "baz"}
  </code></pre>
  
  <p>I'm looking for a really clean way of doing this as my current implementation is pretty messy.</p>