title: Get the user ID in WooCommerce Subscriptions
body: <p>I am using WooCommerce Subscriptions plugin and I am trying to get the customer or userid of a given wc_subscription. </p>
  
  <p>Here is the code I have been using but fails:</p>
  
  <pre><code>add_action( 'woocommerce_scheduled_subscription_trial_end', 'registration_trial_expired', 100 );
  function registration_trial_expired( $wc_subscription ) {
      mail("example@gmail.com", "Expired", "Someone's order has expired");
      $userid = $wc_subscription-&gt;customer_user;
      mail("example@gmail.com", "Expired", "Someone's order has expired with customer".$userid);
  ...
  }
  </code></pre>
  
  <p>I thought <code>$wc_subscription-&gt;customer_user</code> will have the userid but it is empty. In fact stops the code from continuing.</p>
  
  <p>How can I get the user ID with <code>$wc_subscription</code>?</p>