title: Why is 'transform_keys' method undefined?
body: <p>This example is taken directly from the <a href="https://ruby-doc.org/core-2.4.1/Hash.html#method-i-transform_keys" rel="nofollow noreferrer">Ruby 2.4.1 documentation</a>, and I can confirm I am running 2.4.1:</p>
  
  <pre><code>({a: 1, b: 2, c: 3}).transform_keys {|k| k.to_s}
  </code></pre>
  
  <p>When I execute it, I receive the following error:</p>
  
  <pre><code>NoMethodError: undefined method `transform_keys' for {:a=&gt;1, :b=&gt;2, :c=&gt;3}:Hash
  </code></pre>
  
  <p>Why is the <code>transform_keys</code> method not defined?</p>