title: Check if an array is subset of another array in Ruby
body: <p>How can I check whether one array is a subset of another array, regardless of the order of elements?</p>
  
  <pre><code>a1 = [3, 6, 4]
  a2 = [1, 2, 3, 4, 5, 6, 7, 8, 9]
  
  ...?
  
  a1 is a subset of a2
  </code></pre>