title: Rails: error when adding two ActiveSupport::Duration objects
body: <p>In a Rails console (Rails 5.0.1), I have an <code>ActiveSupport::Duration</code> object representing a time interval, in this case 12 minutes:</p>
  
  <p><code>x = ActiveSupport::Duration.parse("P0Y0M0DT0H12M")</code></p>
  
  <p>I'd like to add a random amount of minutes to it, so I create another <code>ActiveSupport::Duration</code> object:</p>
  
  <p><code>y = ActiveSupport::Duration.parse("P0Y0M0DT0H3M")</code></p>
  
  <p>According to <a href="http://api.rubyonrails.org/classes/ActiveSupport/Duration.html#method-i-2B" rel="nofollow noreferrer">the docs</a>, I would expect to be able to add them simply using a <code>+</code> operator and do <code>x+y</code>, but that results in:</p>
  
  <p><code>NoMethodError: undefined method '+' for {:years=&gt;0, :months=&gt;0, :days=&gt;0, :hours=&gt;0, :minutes=&gt;12}:Hash</code></p>
  
  <p>Where am I going wrong? Why does it think they are Hashes? </p>