Update credit_card_gateway.rb

One should be able to configure 'fail_on_duplicate_payment_method' for updates - otherwise someone could create a credit card, then 'update' their credit card and enter in a duplicate. This patch was tested and works with:


      @tr_data = Braintree::TransparentRedirect.update_customer_data(
          :redirect_url => shop_braintree_update_callback_url,
          :customer_id => current_user.id,
          :customer => {
              :id => current_user.id,
              :credit_card => {
                  :options => {
                      :update_existing_token => billing_info[:cc_token],
                      :verify_card => true,
                      :fail_on_duplicate_payment_method => true
                  },
                  :billing_address => {
                      :options => {
                          :update_existing => true
                      }
                  }
              }
          }
      )