title: Convert array of 2-element arrays into a hash, where duplicate keys append additional values
body: <h2>For example</h2>
  
  <p>Given an array:</p>
  
  <pre><code>array = [[:a,:b],[:a,:c],[:c,:b]]
  </code></pre>
  
  <p>Return the following hash:</p>
  
  <pre><code>hash = { :a =&gt; [:b,:c] , :c =&gt; [:b] }
  </code></pre>
  
  <p><code>hash = Hash[array]</code> overwrites previous associations, producing:</p>
  
  <pre><code>hash = { :a =&gt; :c , :c =&gt; :b }
  </code></pre>