title: Hash missing value, throwing exception
body: <p>I have a hash which looks like this:</p>
  
  <pre><code>items:
      item:
          attribute_a: cheese
          attribute_b: bacon
      item:
          attribute_a: salmon
      item:
          attribute_a: mushrooms
          attribute_b: steak
  </code></pre>
  
  <p>I would like to get the value of <code>attribute_b</code>, I'm using the following:</p>
  
  <pre><code>if (result['attribute_b'])
    // do something
  end
  </code></pre>
  
  <p>However if <code>attribute_b</code> is missing, I get an error:</p>
  
  <pre><code>The Identifier specified does not exist, undefined method '[] for nil:NilClass'
  </code></pre>
  
  <p>What is the (best) correct way to check if <code>attribute_b</code> exists?</p>