title: Hash has_key? syntax
body: <p>I have three Hashes:</p>
  
  <pre><code>a = {key:"value"}
  b = {"key"=&gt;"value"}
  c = {:key=&gt;"value"}
  </code></pre>
  
  <p>And these results:</p>
  
  <pre><code>a.has_key?("key") # =&gt; false
  b.has_key?("key") # =&gt; true
  c.has_key?("key") # =&gt; false
  </code></pre>
  
  <p>My questions are:</p>
  
  <ul>
  <li>Why? What is this behavior?</li>
  <li>How can I get <code>a</code>, <code>b</code> and <code>c</code> to return <code>true</code> using the same syntax for all of them?</li>
  </ul>