title: Stripe, checking if coupon exists before creating subscription
body: <p>I am trying to check to see if a coupon exists before creating a subscription. It basically allows me to capture the card info without charging the card until the end of the trial period.</p>
  
  <p>If it exists, I don't want to create it. If it doesnt, it needs to be created.</p>
  
  <p>However, it keeps throwing an error when I try this:</p>
  
  <pre><code>// Check if coupon is valid
  $coupon_check = \Stripe\Coupon::retrieve('free-period');
  
  if (!$coupon_check-&gt;valid) {
    $coupon = \Stripe\Coupon::create(array(
      'duration' =&gt; 'once',
      'id' =&gt; 'free-period',
      'percent_off' =&gt; 100,
    ));
  }
  </code></pre>