Wirecard: Fix instances of 'amex' - will be 'american_express'

Fix instances of `amex` card brand in the Wirecard gateway.

Will be returned as `american_express` by CreditCard#brand. (see: [[1]](https://github.com/Shopify/active_merchant/blob/1e2bcde60d3a461656a046275012cd09fb980c1b/lib/active_merchant/billing/credit_card.rb#L81) [[2]](https://github.com/Shopify/active_merchant/blob/1e2bcde60d3a461656a046275012cd09fb980c1b/lib/active_merchant/billing/credit_card_methods.rb#L9))