Proposed Fix for 5237

This is a proposed fix for #5237. Please note this pull request is **not ready** to pull into master.

I've overridden the `is_available` method within the PayPal gateway class which makes the most semantic sense to me. 

I am, however, not able to pass through the order total to this class. If you look at this line: `if ( get_woocommerce_currency() == $key && 2 > 1 ) {` you can see that I've left the `2 > 1` as a placeholder. Is there an easy way to pass in the order total? Or should this be done with with the `woocommerce_available_payment_gateways` hook?