Stop stealing documents

Stealing documents can have adverse effects since there might be JavaScript
node objects referencing the xml document (using the _private member of the
xmlElement structure).  If we steal such a document, things can break in
many places since ownership becomes inconsistent.

src/node_libxslt.cc

@@ -15,22 +15,16 @@
 
 using namespace v8;
 
-// Assume ownership of the input document at the libxml level.
-// The libxmljs object will be modified so it now represents an empty document.
-static xmlDoc* stealDocument(Local<Value> input) {
+static xmlDoc* copyDocument(Local<Value> input) {
     libxmljs::XmlDocument* docWrapper =
         Nan::ObjectWrap::Unwrap<libxmljs::XmlDocument>(input->ToObject());
     xmlDoc* stylesheetDoc = docWrapper->xml_obj;
-    xmlDoc* dummyDoc = xmlNewDoc((const xmlChar*)"1.0");
-    stylesheetDoc->_private = NULL;
-    dummyDoc->_private = docWrapper;
-    docWrapper->xml_obj = dummyDoc;
-    return stylesheetDoc;
+    return xmlCopyDoc(stylesheetDoc, true);
 }
 
 NAN_METHOD(StylesheetSync) {
     Nan::HandleScope scope;
-    xmlDoc* doc = stealDocument(info[0]);
+    xmlDoc* doc = copyDocument(info[0]);
     xsltStylesheetPtr stylesheet = xsltParseStylesheetDoc(doc);
     // TODO fetch actual error.
     if (!stylesheet) {
@@ -83,7 +77,7 @@ class StylesheetWorker : public Nan::AsyncWorker {
 
 NAN_METHOD(StylesheetAsync) {
     Nan::HandleScope scope;
-    xmlDoc* doc = stealDocument(info[0]);
+    xmlDoc* doc = copyDocument(info[0]);
     Nan::Callback *callback = new Nan::Callback(info[1].As<Function>());
     StylesheetWorker* worker = new StylesheetWorker(doc, callback);
     Nan::AsyncQueueWorker(worker);