title: ruby add key to hash without value
body: <p>Is it possible to add a key to hash without value? So I've created a hash(@j) and have a method:</p>
  
  <pre><code>def add(hash)
  @j.merge!(hash) 
  end
  </code></pre>
  
  <p>How do I make it possible to add keys, without value, example</p>
  
  <pre><code>@j.add('fish')
  puts @j.entries
  puts @j.keywords
  
  =&gt; {'fish' =&gt; nil}
  =&gt; fish
  </code></pre>
  
  <p>My current code allows me to add key and value like this:</p>
  
  <pre><code> @j.add('fish' =&gt; 'animal') 
  </code></pre>
  
  <p>But not if it were like above.... only the key</p>