title: Merge object with arrays
body: <p>I'd like to merge objects with arrays but it seems not working as expected.</p>
  
  <p>I'd like to concat arrays but keep unique items by "label".</p>
  
  <p><strong>First object :</strong></p>
  
  <pre><code>var object1 = {
    index: [
      {
        label: 'Ajouter',
        url: '/_add/:model',
        icon: 'add',
        isMain: true
      },
      {
        label: 'Exporter en CSV',
        url: '/_export/:model',
        icon: 'cloud_download'
      },
      {
        label: 'Accès à l\'API',
        url: '/api/_index/:model',
        target: '_blank',
        icon: 'link'
      }
    ],
    add: [
      {
        label: 'Retour',
        url: '/_index/:model',
        icon: 'arrow_back'
      }
    ],
    edit: [
      {
        label: 'Retour',
        icon: 'arrow_back',
        url: '/_index/:model'
      },
      {
        label: 'Supprimer',
        icon: 'delete',
        url: '/_delete/:model/:id'
      }
    ]
  };
  </code></pre>
  
  <p><strong>Second object :</strong></p>
  
  <pre><code>var object2 = _.mergeWith(_.clone(object1), {
    index: [
      {
        label: 'Explorateur ',
        url: '/file/_folder-explorer/uploads',
        target: '_blank',
        icon: 'folder_open'
      },
      {
        label: 'Ajout multiple ',
        url: '/file/_add-multiple',
        icon: 'library_add'
      }
    ],
    edit: [
      {
        label: 'Retour',
        icon: 'arrow_back',
        url: '/file/_index'
      }
    ]
  }, customizerMergeActions);
  
  function customizerMergeActions(objValue, srcValue) {
    if (_.isArray(objValue)) {
      return objValue.concat(srcValue);
    }
  }
  </code></pre>
  
  <p><strong>Issue :</strong> In the "edit" key I got the same object "label: 'Retour'", I'd like to keep the last one...</p>