title: `Array#join` on nested arrays only
body: <p>I have an array of arrays. The second element of each subarray is further an array:</p>
  
  <pre><code>arr = [
    ["val1", ["The cat", "3"]],
    ["val2", ["Big", "Another", "6"]],
    ["val3", ["343"]]
  ]
  </code></pre>
  
  <p>I would like to convert the array so that the arrays at the third nested level are each joined into one string separated by a space and comma like this:</p>
  
  <pre><code>[["val1", "The cat, 3"], ["val2", "Big, Another, 6"], ["val3", "343"]]
  </code></pre>
  
  <p>How do I call <code>join</code> on just the nested arrays?</p>