title: Laravel using UNION in query builder
body: <p>I have an SQL query that works fine and I'm trying to convert into fluent::</p>
  
  <pre><code>SELECT DISTINCT tags.tag
    FROM tags, items
    WHERE tags.taggable_type = 'Item'
    AND items.item_list_id = '1'
  UNION
  SELECT DISTINCT tags.tag
    FROM tags, itemlists
    WHERE tags.taggable_type = 'ItemList'
    AND itemlists.id = '1'
  </code></pre>
  
  <p>This is what I have so far in fluent, it all seems right as far as I can tell from the docs and the individual queries both work on their own, it's just when I UNION them it throws an error:</p>
  
  <pre><code>   $itemTags = Tag::join('items', 'items.id', '=', 'tags.taggable_id')
                  -&gt;select('tags.tag')
                  -&gt;distinct()
                  -&gt;where('tags.taggable_type', '=', 'Item')
                  -&gt;where('items.item_list_id', '=', $itemList-&gt;id);
  
      $itemListTags = Tag::join('itemlists', 'itemlists.id', '=', 'tags.taggable_id')
                  -&gt;select('tags.tag')
                  -&gt;distinct()
                  -&gt;where('tags.taggable_type', '=', 'ItemList')
                  -&gt;where('itemlists.id', '=', $itemList-&gt;id);
  // the var_dump below shows the expected results for the individual queries
  // var_dump($itemTags-&gt;lists('tag'), $itemListTags-&gt;lists('tag')); exit;
      return      $itemTags
                  -&gt;union($itemListTags)
                  -&gt;get();
  </code></pre>
  
  <p>I get the following error when I run it (I've also swapped from Ardent back to Eloquent on the model in case that made a difference - it doesn't):</p>
  
  <pre><code>Argument 1 passed to Illuminate\Database\Query\Builder::mergeBindings() must be an instance of Illuminate\Database\Query\Builder, instance of LaravelBook\Ardent\Builder given, called in path/to/root\vendor\laravel\framework\src\Illuminate\Database\Query\Builder.php on line 898 and defined 
  </code></pre>