SecurePayAu: fix credit card check

- Some ActiveMerchant users have their own credit card object that they
  pass to ActiveMerchant so we shouldn't be checking for
ActiveMerchant::Billing::CreditCard explicitly but check that it looks
like a credit card instead (having a number method is a good check).

lib/active_merchant/billing/gateways/nab_transact.rb

@@ -59,7 +59,7 @@ def test?
       end
 
       def purchase(money, credit_card_or_stored_id, options = {})
-        if credit_card_or_stored_id.is_a?(ActiveMerchant::Billing::CreditCard)
+        if credit_card_or_stored_id.respond_to?(:number)
           #Credit card for instant payment
           commit :purchase, build_purchase_request(money, credit_card_or_stored_id, options)
         else

lib/active_merchant/billing/gateways/secure_pay_au.rb

@@ -64,7 +64,7 @@ def test?
       end
 
       def purchase(money, credit_card_or_stored_id, options = {})
-        if credit_card_or_stored_id.is_a?(ActiveMerchant::Billing::CreditCard)
+        if credit_card_or_stored_id.respond_to?(:number)
           requires!(options, :order_id)
           commit :purchase, build_purchase_request(money, credit_card_or_stored_id, options)
         else

test/unit/gateways/secure_pay_au_test.rb

@@ -58,6 +58,18 @@ def test_failed_purchase
     assert_equal "CARD EXPIRED", response.message
   end
 
+  def test_purchase_with_stored_id_calls_commit_periodic
+    @gateway.expects(:commit_periodic)
+
+    @gateway.purchase(@amount, "123", @options)
+  end
+
+  def test_purchase_with_creditcard_calls_commit_with_purchase
+    @gateway.expects(:commit).with(:purchase, anything)
+
+    @gateway.purchase(@amount, @credit_card, @options)
+  end
+
   def test_successful_authorization
     @gateway.expects(:ssl_post).returns(successful_authorization_response)
 
@@ -454,4 +466,4 @@ def successful_refund_response
   def failed_refund_response
     %(<?xml version="1.0" encoding="UTF-8" standalone="no"?><SecurePayMessage><MessageInfo><messageID>6bacab2b7ae1200d8099e0873e25bc</messageID><messageTimestamp>20102807071248484000+600</messageTimestamp><apiVersion>xml-4.2</apiVersion></MessageInfo><RequestType>Payment</RequestType><MerchantInfo><merchantID>CAX0001</merchantID></MerchantInfo><Status><statusCode>000</statusCode><statusDescription>Normal</statusDescription></Status><Payment><TxnList count="1"><Txn ID="1"><txnType>4</txnType><txnSource>23</txnSource><amount>101</amount><currency>AUD</currency><purchaseOrderNo>269061</purchaseOrderNo><approved>No</approved><responseCode>134</responseCode><responseText>Only $1.00 available for refund</responseText><thinlinkResponseCode>300</thinlinkResponseCode><thinlinkResponseText>000</thinlinkResponseText><thinlinkEventStatusCode>999</thinlinkEventStatusCode><thinlinkEventStatusText>Error - Transaction Already Fully Refunded/Only $x.xx Available for Refund</thinlinkEventStatusText><settlementDate/><txnID/><CreditCardInfo><pan>444433...111</pan><expiryDate>09/11</expiryDate><cardType>6</cardType><cardDescription>Visa</cardDescription></CreditCardInfo></Txn></TxnList></Payment></SecurePayMessage>)
   end
-end
+end