Resolve Nokogiri deprecation warning

This fixes the following deprecation warning:

```
worldpay_cnp-0.1.0/lib/worldpay_cnp/xml/nokogiri.rb:36: warning: Passing a Node as the second parameter to Node.new is deprecated. Please pass a Document instead, or prefer an alternative constructor like Node#add_child. This will become an error in a future release of Nokogiri.
```

We are now passing the document along with the parent element. The serializer will handle the case of setting the parent to the document, when the parent is nil due to the recursive nature of the method calls.

This deprecation was included in release 1.13.0 https://github.com/sparklemotion/nokogiri/releases/tag/v1.13.0

For reference: sparklemotion/nokogiri#975 and sparklemotion/nokogiri#2401

lib/worldpay_cnp/xml/nokogiri.rb

@@ -26,16 +26,16 @@ class Serializer
         include XML::Serializer
 
         def call(hash)
-          ::Nokogiri::XML::Document.new.tap { |d| add_xml_elements!(d, hash) }.to_s
+          ::Nokogiri::XML::Document.new.tap { |d| add_xml_elements!(d, nil, hash) }.to_s
         end
 
         private
 
-        def attributes_or_elements!(parent, key, value)
+        def attributes_or_elements!(document, parent, key, value)
           return parent[attribute_name(key)] = text_with(value) if attribute?(key)
-          element = ::Nokogiri::XML::Element.new(key.to_s, parent)
+          element = ::Nokogiri::XML::Element.new(key.to_s, document)
           parent.add_child(element)
-          add_xml_elements!(element, value)
+          add_xml_elements!(document, element, value)
         end
 
         def insert_text!(element, text)

lib/worldpay_cnp/xml/serializer.rb

@@ -7,20 +7,21 @@ def call(hash)
 
       private
 
-      def attributes_or_elements!(parent, key, value)
+      def attributes_or_elements!(document, parent, key, value)
         raise NotImplementedError
       end
 
       def insert_text!(element, text)
         raise NotImplementedError
       end
 
-      def add_xml_elements!(parent, obj)
+      def add_xml_elements!(document, parent, obj)
+        parent = document if parent.nil?
         case obj
         when Hash
-          obj.each { |key, value| attributes_or_elements!(parent, key, value) }
+          obj.each { |key, value| attributes_or_elements!(document, parent, key, value) }
         when Array
-          obj.each { |value| add_xml_elements!(parent, value) }
+          obj.each { |value| add_xml_elements!(document, parent, value) }
         else
           insert_text!(parent, obj)
         end