Add `currency` attribute to customer

lib/stripe_mock/data.rb

@@ -32,6 +32,7 @@ module StripeMock
         delinquent: false,
         discount: nil,
         account_balance: 0,
+        currency: nil,
         sources: {
           object: "list",
           total_count: sources.size,

lib/stripe_mock/request_handlers/helpers/subscription_helpers.rb

@@ -30,6 +30,7 @@ module StripeMock
         charges[id] = Data.mock_charge(:id => id, :customer => cus[:id], :amount => sub[:plan][:amount])
         cus[:subscriptions][:total_count] = (cus[:subscriptions][:total_count] || 0) + 1
         cus[:subscriptions][:data].unshift sub
+        cus[:currency] = sub[:plan][:currency]
       end
 
       def delete_subscription_from_customer(cus, subscription)