Validate a class Before Use it

Current implementation of `WC_Shipping` and `WC_Payment_Gateway` to add/register a shipping method and gateway is via Wordpress filter. We are don't know what is passed there, so now we must validate each class name passed there.

I am start to implement a validation using ReflectionClass. Another options is using typehinting. I am also add phpunit to test that. What do you think?