title: Setting proration_behavior to none in Stripe
body: <p>I am trying to update my subscription in such a way to remove the prorartion behaviour. Here's what  I did, (It didn't work and while printing the $subscription_object  I didn't see any proration_behavior field despite what it is written in the API documentation of a subscription object)</p>
  <pre><code> $subscription_obj = \Stripe\Subscription::retrieve($checkout_session-&gt;subscription); 
  \Stripe\Subscription::update($subscription_obj-&gt;id, [
          'proration_behavior' =&gt; 'none',
        ]);
  </code></pre>
  <p>I also tried :</p>
  <pre><code>$subscription_obj-&gt;proration_behavior = 'none';
  $subscription_obj-&gt;save();
  </code></pre>