Response object returned after a REST call to Amazon service.
XML input is in string format
Return Nokogiri::XML::Document object.
Return error message.
Return error code
Return the first item (Amazon::Element)
Return true if response has an error.
Return true if request is valid.
Return current page no if :item_page option is when initiating the request.
Return an array of Amazon::Element item objects.
Return total pages.
Return total results.
[Validate]