Ensure cache is cleared on close if checkout is complete

[kiftio]

What are you trying to accomplish?

The last change was to retain the WebView cache on close, so that it can be re-opened quickly, and matches with swift. This updates that a little to ensure we clear the cache on close when the checkout is complete.

Before you deploy

• [x] I have added tests to support my implementation
• [x] I have read and agree with the contributing documentation
• [x] I have read and agree with the code of conduct documentation
• [x] I have updated any documentation related to these changes.
• [x] I have updated the README (if applicable).