fix: do not serialize recurly_stub objects in body payload

Including the string representation of the fetched stub object is not
proper to send to the API as it is an improper value for that tag.

Author: cbarton

Tests/Recurly/Subscription_Test.php

@@ -279,7 +279,6 @@ public function testUpdateShippingAddressXml() {
           <quantity>1</quantity>
         </subscription_add_on>
         <subscription_add_on>
-          <item>&lt;Recurly_Stub[item] href=https://api.recurlyqa.com/v2/items/mockitem&gt;</item>
           <external_sku>tester-sku</external_sku>
           <add_on_code>mockitem</add_on_code>
           <unit_amount_in_cents>199</unit_amount_in_cents>
@@ -476,7 +475,6 @@ public function testUpdateSubscriptionWithAddOns() {
           <quantity>1</quantity>
         </subscription_add_on>
         <subscription_add_on>
-          <item>&lt;Recurly_Stub[item] href=https://api.recurlyqa.com/v2/items/mockitem&gt;</item>
           <external_sku>tester-sku</external_sku>
           <add_on_code>mockitem</add_on_code>
           <unit_amount_in_cents>199</unit_amount_in_cents>

lib/recurly/traits/xml_doc.php

@@ -178,6 +178,8 @@ public function populateXmlDoc(&$doc, &$node, &$obj, $nested = false)
       // doesn't extend Recurly_Resource we should add an interface for this.
       if ($val instanceof Recurly_CurrencyList || $val instanceof Recurly_CustomFieldList) {
         $val->populateXmlDoc($doc, $node);
+      } else if ($val instanceof Recurly_Stub) {
+        continue;
       } else if ($val instanceof Recurly_Resource) {
         $attribute_node = $node->appendChild($doc->createElement($key));
         $this->populateXmlDoc($doc, $attribute_node, $val, true);