Fix errors caused by demo payment methods

[nerrad]

This was introduced in #1349 . The way the demo payment method registration was setup, the react nodes were root native dom elements (<div>, <img />) so when React.cloneElement was called on them to pass in the checkout props, these props were then applied to the native nodes which in turn caused the React warnings due to camel-case props (isActive, checkoutData etc). The solution was to modify the demo elements so that they were custom react nodes (which is what actual payment methods will be doing anyways).

To Test

View the console for the checkout block in the editor or on the frontend and there should be no warnings in the console.

[haszari]

Tested and confirmed this fixes the js errors.

Code changes look good to me – thanks for jumping on this @nerrad 🎉