just return the ruby object of node if one exists

the fix in the previous commit is overly complicated. the if conditional
originaly tested whether the passed document has a ruby object in
'_private' which is unnecessary since we return the node's ruby object
if one exists. This commit moves the conditional to the beginning of the
function and simplifies it.a new object will be created and attach to
the document, otherwise.

ext/nokogiri/xml_namespace.c

@@ -37,13 +37,10 @@ static VALUE href(VALUE self)
 VALUE Nokogiri_wrap_xml_namespace(xmlDocPtr doc, xmlNsPtr node)
 {
   VALUE ns, document, node_cache;
-  nokogiriTuplePtr node_has_a_document;
 
-  if (doc->type == XML_DOCUMENT_FRAG_NODE) doc = doc->doc;
-  node_has_a_document = DOC_RUBY_OBJECT_TEST(doc);
+  if (node->_private) return (VALUE)node->_private;
 
-  if(node->_private && node_has_a_document)
-    return (VALUE)node->_private;
+  if (doc->type == XML_DOCUMENT_FRAG_NODE) doc = doc->doc;
 
   ns = Data_Wrap_Struct(cNokogiriXmlNamespace, 0, 0, node);