title: WP_Query doesn't order by price meta value
body: <p>I have this wp_query $args to get products from woocommerce querying them by some $_GET parameters I used for filters. 
  Actually my problem is that sorting by price isn't working at all. I used this kind of attributes many times but actually here isn't working.
  I paste here my code.</p>
  
  <pre><code>if ($_GET['filter_pietre'] != -1 &amp;&amp; $_GET['filter_pietre'] != NULL) {
      $pietre_operator = 'IN';
  } else {
      $pietre_operator = 'NOT IN';
  }
  if ($_GET['filter_metals'] != -1 &amp;&amp; $_GET['filter_metals'] != NULL) {
      $metals_operator = 'IN';
  } else {
      $metals_operator = 'NOT IN';
  }
  if ($_GET['filter_finitura'] != -1 &amp;&amp; $_GET['filter_finitura'] != NULL) {
      $finishes_operator = 'IN';
  } else {
      $finishes_operator = 'NOT IN';
  }
  if ($_GET['filter_coloresmalto'] != -1 &amp;&amp; $_GET['filter_coloresmalto'] != NULL) {
      $pa_coloresmalto = 'IN';
  } else {
      $pa_coloresmalto = 'NOT IN';
  }
  if ($_GET['filter_ispirazione'] != -1 &amp;&amp; $_GET['filter_ispirazione'] != NULL) {
      $pa_ispirazione = 'IN';
  } else {
      $pa_ispirazione = 'NOT IN';
  }
  $params = array(
          'posts_per_page' =&gt; -1,
          'post_type' =&gt; 'product',
          'meta_query' =&gt; array(
              //filters
              'relation' =&gt; 'AND',
              array(
                  'key' =&gt; '_stock_status',    
                  'value' =&gt; 'instock'
              ),
              array(
                  'meta_key' =&gt; '_price',
                  'orderby'  =&gt; 'meta_value_num',
                  'order' =&gt; 'ASC',
                  'type' =&gt; 'NUMERIC'
              ),
          ),
          'tax_query' =&gt; array(
              'relation' =&gt; 'AND',
              array(
                  'taxonomy' =&gt; 'product_cat',
                  'field' =&gt; 'id',
                  'terms' =&gt; $term_id
              ),
              array(
                  'taxonomy' =&gt; 'pa_pietre',
                  'terms' =&gt; $_GET['filter_pietre'],
                  'field' =&gt; 'slug',
                  'operator' =&gt; $pietre_operator
              ),
              array(
                  'taxonomy' =&gt; 'pa_metals',
                  'terms' =&gt; $_GET['filter_metals'],
                  'field' =&gt; 'slug',
                  'operator' =&gt; $metals_operator
              ),
              array(
                  'taxonomy' =&gt; 'pa_finishes',
                  'terms' =&gt; $_GET['filter_finitura'],
                  'field' =&gt; 'slug',
                  'operator' =&gt; $finishes_operator
              ),
              array(
                  'taxonomy' =&gt; 'pa_coloresmalto',
                  'terms' =&gt; $_GET['filter_coloresmalto'],
                  'field' =&gt; 'slug',
                  'operator' =&gt; $pa_coloresmalto
              ),
              array(
                  'taxonomy' =&gt; 'pa_ispirazione',
                  'terms' =&gt; $_GET['filter_ispirazione'],
                  'field' =&gt; 'slug',
                  'operator' =&gt; $pa_ispirazione
              )
          ),
      );
  $wc_query = new WP_Query($params);
  </code></pre>
  
  <p>Actually sorting by any other attributes works properly but not price.
  Thanks in advance. </p>