title: Woocommerce shortcode orderby price is not working
body: <p>I have spend hours yesterday solving my problem but it didn't work. 
  I want to sort my product of a specific brand on price (low to high), all other sorting options work fine title, id, etc.. But my self declared $arg regular_price won't work.</p>
  
  <p>I have added this code to functions.php in my theme map:</p>
  
  <pre><code>add_filter( 'woocommerce_shortcode_products_query', 'woocommerce_shortcode_products_orderby' );
  function woocommerce_shortcode_products_orderby( $args ) 
  {  $standard_array = array('menu_order','title','date','rand','id');    
  if( isset( $args['orderby'] ) &amp;&amp; !in_array( $args['orderby'], $standard_array ) ) 
  {  $args['regular_price'] = $args['orderby'];       
     $args['orderby']  = 'meta_value_num'; }  
    return $args;}
  </code></pre>
  
  <p>And I use this shortcode :</p>
  
  <pre><code>[product_attribute attribute="merk" filter="bait footwear" per_page="80" columns="4" orderby="regular_price" order="asc"]
  </code></pre>
  
  <p>The result is here:
  <a href="http://vintagekledingwinkel.nl/aanbiedingen/bait-footwear/" rel="nofollow">http://vintagekledingwinkel.nl/aanbiedingen/bait-footwear/</a></p>
  
  <p>What am i missing?</p>