title: Hash remove all except specific keys
body: <p>I would like to remove every key from a hash except a given key.</p>
  <p>For example:</p>
  <pre><code>{
   &quot;firstName&quot;: &quot;John&quot;,
   &quot;lastName&quot;: &quot;Smith&quot;,
   &quot;age&quot;: 25,
   &quot;address&quot;:
   {
       &quot;streetAddress&quot;: &quot;21 2nd Street&quot;,
       &quot;city&quot;: &quot;New York&quot;,
       &quot;state&quot;: &quot;NY&quot;,
       &quot;postalCode&quot;: &quot;10021&quot;
   },
   &quot;phoneNumber&quot;:
   [
       {
         &quot;type&quot;: &quot;home&quot;,
         &quot;number&quot;: &quot;212 555-1234&quot;
       },
       {
         &quot;type&quot;: &quot;fax&quot;,
         &quot;number&quot;: &quot;646 555-4567&quot;
       }
   ]
  }
  </code></pre>
  <p>I want to remove everything except &quot;firstName&quot; and/or &quot;address&quot;.</p>