title: Matter JS - event.pairs array is blank on collision
body: <p>I'm using <a href="http://brm.io/matter-js" rel="nofollow noreferrer">MatterJS</a> to animate some objects on a canvas. At one point in the animation there is a collision between two of my objects. I'm successfully detecting the event with the below code:</p>
  
  <pre><code>Events.on(engine, 'collisionStart', function(event) {
  
      console.log(event)
  
  });
  </code></pre>
  
  <p>What i'm trying to identify is which two objects are colliding. When I console.log event.pairs I get an empty array. I would have thought it would have returned an array of the two objects that are colliding. Where have I gone wrong?</p>