title: Lodash union of arrays of objects
body: <p>I'd like to use the <code>_.union</code> function to create a union of two arrays of objects. Union works with arrays of primitives only as it uses === to examine if two values are equal.</p>
  
  <p>I'd like to compare objects using a key property: objects with the same key property would be regarded equal. Is there a nice functional way to achieve that ideally using lodash?</p>