title: Verify cancel_return parameters
body: <p>When I click the cancel link in PayPal and am redirected to my cancel_url, it is passed the following parameters (some values removed).</p>
  
  <pre><code>cmd=_flow&amp;
  myAllTextSubmitID=&amp;
  miniPager=&amp;
  currentSession=&amp;
  pageState=login&amp;
  currentDispatch=&amp;
  email_recovery=false&amp;
  password_recovery=false&amp;
  login_email=&amp;
  login_password=&amp;
  private_device_checkbox_flag=on&amp;
  SESSION=&amp;
  CONTEXT=&amp;
  cmd=_flow&amp;
  id=&amp;
  close_external_flow=false&amp;
  external_close_account_payment_flow=payment_flow&amp;
  cancel_return=&amp;
  auth=&amp;
  form_charset=UTF-8&amp;
  external_remember_me_read_cookie_ids=&amp;
  flow_name=xpt%2FCheckout%2Fwps%2FLogin&amp;
  fso=
  </code></pre>
  
  <p>Is there any way to verify that this is a legitimate cancelation similar to how IPN verify works?</p>