Clicking cancel link in 3DS2 challenge displays shipping step in modal

[chrismshea]

PRs: #253

Steps to reproduce:

1. I am using Config 1
2. Logged in as a customer on Magento 2
3. I add a non subscription item to the cart, and proceed to checkout
4. On the payment step use a new card using 2303779999000424 which is a 2.1 Payment Flow / Challenge card
5. Click Place Order and the Challenge is displayed.
6. Click cancel in the upper right corner of the iframe
7. Shipping Step is displayed in the iframe

Expectation:

When clicking cancel in the 3DS challenge iframe the iframe should close.