title: Multiple order by in WooCommerce
body: <p>I want to order my products in category page by average rating (DESC) and then by price (ASC).</p>
  
  <pre><code>| id  |  avgrating   |   price   |
  |  1  |      4       |     10    |
  |  2  |      4       |      5    |
  |  3  |      5       |      7    |
  </code></pre>
  
  <p>Order: 3, 2, 1.</p>
  
  <p>So I tried with:</p>
  
  <pre><code>$args['meta_key'] = '_wc_average_rating';
  $args['orderby']  = array(
      'meta_value_num' =&gt; 'DESC',
      'price' =&gt; 'ASC',
  );
  </code></pre>
  
  <p>But they aren't ordered (also) by price. I also replaced <code>price</code> with <code>_price</code>, same result.</p>
  
  <p>I'm using latest version of WordPress (4.8) and WooCommerce (3.0.8).</p>
  
  <p><strong>Edit:</strong></p>
  
  <p>If I use:</p>
  
  <pre><code>$args['meta_key'] = '_wc_average_rating';
  $args['orderby']  = array(
      'meta_value_num' =&gt; 'DESC',
      'ID'             =&gt; 'DESC',
  );
  </code></pre>
  
  <p>Order works as excepted, DESC by average rating and then DESC by ID. So, I have to change <code>ID</code> with <code>price</code> but I can't make it work.</p>